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Marine Electrical Equipmentand Practice

Second edition

H. D. McGeorgC, CEng, FIMarE, MRINA

£1IJ T T E R W O R T _ HITIE: I N E M ANN

Buticrworth-l IcincmannLinacre House. Jordan H i l l , Oxford OX2 8DP225 Wildwood Avenue, Wohurn, MA 01801-2041A division of Reed Educational and Professional Publishing Ltd

A member of the Reed Elsevier pic group

"OXFORD AUCKLAND BOSTONJOHANNESBURG MELBOURNE NEW DELHIFirsl published by Standlbrd Mari t ime Lid 1986Second edition 1993Reprinied 1995, 1997. 1999 (twice). 2000

© H. David McGeorge 1986. 1993

All r ights reserved. No pan of this publicationmay be reproduced in any material form (includingphotocopying or storing in any medium by electronicmeans and whether or not transiently on incidenta l lyto some other use of this publication) withoul thewritten permission of the copyright holder exceptin accordance with the provisions of the Copyright,Designs and Patents Act 1998 or under the terms of alicence issued by the Copyright Licencing Agency Lid.90Touenham Court Road, London, England WIPOLP.Applications for the copyright holder's wri t ten permissionto reproduce any pan of this publication should be addressedto the publishers.

British Library Cataloguing in Publication DataMcGeorge, H.D.

Marine Electrical Equipment andPractice. - 2Rev.edI. Title

- 623.8503

ISBN 07506 16474

Typeset by Vision Typesetting, ManchesterPrinted and bound in Great Britain by Athensum Press Ltd,Gateshead, Tyne & Wear

PLANT Arot ivun TITU TXVT wt H.IUJM. »VTTTJIWO*T»».HUWJIA>-«

wii fAT KM ITCV TO n_vn AW CAM ro« A nu.

Contents

Preface

1 Batteries and emergency systems2 Electronic equipment3 A.c. generators4 A.c. switchboards and distribution systems5 A.c. motors6 D.c. generators7 D.c. switchboards and distribution systems8 D.c. motors9 Safe electrical equipment for hazardous areas

10 Shaft-driven generators11 Electric propulsion12 Miscellaneous items

Index

'reface

ie object of this book is to provide a description of the various items of ships' electricaluipment, with an explanation of their operating principles.The topics dealt with are those that feature in examination papers for Class 1 and Glass 2:partment of Transport certification. It is hoped that candidates sitting the electricalper or the general engineering knowledge paper will find the book helpful in preparing1 their examinations.This second edition includes new chapters on shaft-driven generators and electricjpulsion, including many new diagrams explaining drive, distribution and controlitems. The treatment of safe electrical equipment has been expanded, and theportunity has been taken to include reference to more specialised published papers onne of the topics discussed.Technical language can be a barrier to the understanding of a subject by then-specialist: an effort has been made to avoid its excessive use, but to explain terms as•y arise. Diagrams are kept as simple as possible so that they can form the basis oftmination sketches. For this reason many diagrams have been redrawn.! am grateful for information received from a number of manufacturers of electricallipment. These include Alcad Ltd; Varta Ltd; NIE-APE-W.H. Allen; Laurence, Scott1 Electromotors Ltd; GEC-Alsthom; Clarke, Chapman & Co. Ltd; British Brownveri Ltd; and Siemens (UK) Ltd. Much information has been obtained from theinsactions of the Institute of Marine Engineers. Thanks to former colleagues R.C. Deani R.E. Lovell for their assistance, and also to Alison Murphy for her help with thegrams.

H.D. McGeorge, CEng, FIMarE, MRINA

CHAPTER ONE

Batteries and Emergency Systems

Lead-acid storage batteries

Each cell of a lead-acid battery contains two interleaved sets of plates, immersed inelectrolyte. Those connected to the positive terminal of a charged cell are of lead peroxide;those connected to the negative terminal are of lead. The simple sketch used here to explainthe discharge and recharge has only one plate of each type (Figure 1.1).

The electrolyte in which tin plates are immersed is a dilute solution of sulphuric acid indistilled water. A characteristic of electrolytes is that they contain ions of the compoundsdissolved in them which can act as current carriers. In this solution, the ions are providedby sulph ir acid (H2SO4) molecules, which split into positively charged hydrogen ions(H*) anc negatively charged sulphate ions (SO4 "'). The separated parts of the moleculeare electrically unbalanced because the split leaves sulphate ions with extra (negative)electrons, and hydrogen ions with an overall positive charge due to the loss of electrons.

Discharge action

During discharge, the hydrogen ions (H *) remove oxygen from the lead peroxide (PbO2)of the positive plates and combine with it to form water (H2O). Loss of oxygen from thelead peroxide reduces it to grey lead (Pb). The water formed by the action dilutes theelectrolyte so that as the cell discharges, the specific gravity (relative density) decreases.Measurement of the specific gravity change with a hydrometer will show the state ofcharge of the cell.

At the negative side of the cell, sulphate ions (SO4 ~ ") combine with the pure lead of thenegative plates to torm a~Ta^er ot white lead sulphate (PbSOJ. The lead sulphate layerincreases during discharge and finally covers the acti vematerial of the plale so that ruHrierreaction is stifled. Syne sulphate also forms_pn the positive plates, but this is not a directpart of the discharge reaction.

A fully~chargecTcell will be capable of producing ^.95 volts on load and the relativedensity of the electrolyte will be at a maximum (say 1 280). After a period of discharge theelectrolyte will be weak due to formation ot water and the plates will be sulphated, with theresult that the voltage on load will drop. Recharging is required when voltage on loaddrops to say 1.8 volts per cell and the relative density is reduced to about 1.120. "*

2 lianenes anil emergency systems

/hydrogen

oxygen

grey

sponge

lead

W

-H

~^s

verit cao

] [

electrolyte

dilute sulphuric acid

H2S04

l/XHbSO, f

lead Hp

sulphatewater

mud space

+

brown

lead

peroxide

(PbO,)

Figure 1.1 Lead—acid cell

Charging

To charge lead-acid batteries, the cell is disconnected from the load and^connected to aji.c. charfiinft supply "f th<" rf"Tect voltage^ The positive of the charging supplyTsconnected '" th<> positive side of the cell, and the negative of the charging supply to thenegative of the cell. Flow of current from the charging source reverses the discharge acTibnoi the cell: thus lead sulphate on the plates is broken down. The sulphate goes back intosolutioni as sulphate ions (SU4 ), leaving the plates as pure lead. Water in the electrolytebreaks down returning hydrogen ions (H^ ) to the solution, and allows the oxygen torecombine with the lead of the positive plate and form lead peroxide (PbO2).

Gas emission

Towards the end of charging and during overcharge, the current flowing into the cellcauses a breakdown or electrolysis of water in the electrolyte, shown by bubbles at thesurface. Both hydrogen and oxygen are evolved and released through cell vent caps intothe battery compartment. There is an explosion risk if hydrogen is allowed to accumulate

Batteries and emergency systems J

(flammable range is 4% to 74% of Tfydfbgeri in~ air). Thus" regulations require goodventilation to remove gas and precautions against naked lights or sparks in enclosedbattery compartments (see below).

Topping up

Batteries suffer water loss due to both gassing and evaporation, with consequent drop inliquid level. There is no loss of sulphuric acid from the^electrolyte Qunless through spillage).Regular checks are made to ensure that liquid level is above the top of the plates anddistilled water is added as necessary. Overfilling will cause the electrolyte to bubble out ofthe vent.

Plate construction

A lead-acid battery is made up of a number of cells, each with a nominal voltage of 2 volts.Thus three cells separated by divisions in a common casing and connected in series makeup a 6 volt battery, and six cells arranged in the same way make a 12 volt battery.

Each cell has, say, seven positive and eight negative plates which are interleaved andarranged alternately positive and negative. Common practice is to have both end platesnegative. Plates are prevented from touching by porous separators of insulating plastic.The design of the plates is such as to give the greatest possible surface area, adequatestrength and good conductivity. The porous paste active material extends plate area togive maximum contact between active material and the electrolyte, and therefore goodcapacity. The oxide paste has little strength and is a poor conductor of electricity so thedeficiencies are made good by a lead-antimony grid into which the paste is pressed.

Electrolyte

Sulphuric acid used to make up electrolyte for lead-acid batteries is, in its concentratedform, a non-conductor of electricity. In solution with water it becomes an electrolytebecause of the breakdown of the H2SO4 molecules into hydrogen (H +) ions and sulphate(SO~ ~) ions which act as current carriers in the liquid.

Concentrated sulphuric acid has a great affinity for water and this, together with theheat evolved when they come into contact, makes the production of electrolyte hazardous.A violent reaction results if water is added to concentrated sulphuric acid. Successful safemixing is only possible if the acid is very slowly added to pure water while stirring. Normallythe electrolyte is supplied ready for use in an acid-resistant container.

Electrolyte is strongly corrosive and will damage the skin as well as materials such aspaint, wood, cloth etc. on which it may spill. It is recommended that electrolyte on the skinbe removed by washing thoroughly (for 15 minutes) with water. Acid-resisting paint mustbe used on battery room decks.

Nickel-cadmium storage batteries

The active materials of positive and negative plates in each cell of a charged nickel-cadmium battery (Figure 1.2) are nickel hydrate and cadmium, respectively. Thechemicals are retained in the supporting structure of perforated metal plates and thedesign is such as to give maximum contact between active compounds andjhe electrolyte.

4 Batteries and emergency systems

non-returnvent

hydrogenoxygen

+

charged

NiO(OH) "

discharged

Ni(OH>2

*"

-̂*.

•MKH

j l|A|i [electrolyte

potassium hydroxidesolution

KOH

K"1" OH~

~—

<^-

charged

~ Cd

discharged

Cd(OH)2

Figure 1.2 Nickel-cadmium cell

The strong alkaline electrolyte is a solution of potassium hydroxide in distilled water(with an addition of lithium). The ions produced in the formation of the potassiumhydroxide solution (K + and OH ~) act as current carriers and take part in an ion transfer.

Discharge action

During discharge the complicated but uncertain action at the positive plates (hydratedoxide of nickel) causes hydroxyl ions (OH ") to be introduced into the electrolyte. As theaction progresses, the nickel hydrate is changed to nickel hydroxide. Simultaneously,hydroxyl ions (OH ~) from the electrolyte form cadmium hydroxide with the cadmium ofthe negative plates. Effectively, the hydroxyl ions (OH ~) move from one set of plates to theother, leaving the electrolyte unchanged. There is no significant change in specific gravitythrough the discharge/charge cycle and the state of charge cannot be found by using ahydrometer.

Charging

A direct current supply for charging is obtained from a.c. mains, through the transformerand rectifier in the battery charger. The positive of the charging supply is connected to thepositive of the cell, and negative to the negative terminal. Flow of current from thecharging source reverses the discharge action. The reactions are complicated but can besummarised by the simplified equation:

Batteries and emergency systems 5

Charged Discharged2NiO(OH)+Cd 2Ni(OH)2H2O + Cd(OH)2

Hydrated Cadmium Nickel Cadmiumoxide of hydroxide hydroxidenickel

Gassing

The gases evolved during charging are oxygen (at the positive plates) and hydrogen (at thenegative plates). Rate of production of gas increases in periods of overcharge. Whenhydrogen in air reaches a proportion of about 4% and up to 74% it constitutes anexplosive mixture. Good ventilation of battery compartments is therefore necessary toremove gas. Equipment likely to cause sparking or arcing must not be located orintroduced into battery spaces. Vent caps are non-return valves, as shown diagrammati-cally (Figure 1.2), so that gas is released but contact by the electrolyte with the atmosphereis prevented. The electrolyte readily absorbs carbon dioxide from the atmosphere anddeterioration results because of the formation of potassium carbonate. For this reason, cellvent caps must be kept closed.

Topping up

Gassing is a consequence of the breakdown of water in the electrolyte. This, together witha certain amount of evaporation, means that topping up with distilled water will benecessary from time to time. High consumption of distilled water would suggestovercharging.

Electrolyte

Potassium hydroxide solution is strongly alkaline and the physical and chemicalproperties of potassium hydroxide closely resemble those of caustic soda (sodiumhydroxide). It is corrosive, so care is essential when topping up batteries or handling theelectrolyte. In the event of skin or eye contact, the remedy is to wash with plenty of cleanwater (for 15 minutes) to dilute and remove the solution quickly. Speed is vital to preventburn damage; and water, which is the best flushing agent, must be readily available.Neutralising compounds (usually weak acids) cannot always be located easily, althoughthey should be available in battery Compartments.

Specific gravity of electrolyte in a Ni-Cd cell is about 1.210 and this does not changewith charge and discharge as in lead-acid cells. However, over a period of time the strengthof the solution will gradually-drop and renewal is necessary at about a specific gravity of1.170.

Containers

The electrolyte slowly attacks glass and various other materials. Containers are thereforeof welded sheet steel which is then nickel plated, or moulded in high-impact polystyrene.Steel casings are preferred when battieres are subject to shock and vibration. Hardwoodcrates are used to keep the cells separate from each other and from the support beneath.Separation is necessary because the positive plate assembly is connected to the steel casing.


Plates

The active materials for nickel-cadmium cells are improved by additions of othersubstances. Positive plates carry a paste made up ini t ia l ly of nickel hydroxide with a smallpercentage of other hydroxides to improve capacity and 20% graphite for betterconductivity. The material is brought to the charged state by passing a current through it,v?hich changes the nickel hydroxide to hydrated nickel oxide, NiO(OH). Performance ofcadmium in the negative plates is improved by addition of 25% iron plus small quantitiesof nickel and graphite.

Active materials may be held in pocket or sintered plates. The former are made up fromnickel plated mild steel strip, shaped to form an enclosing pocket. The pockets areinterlocked at their crimped edges and held in a frame. Electrolyte reaches the activematerials through perforations in the pockets.

Sintered plates are produced by heating (to 900°C) powdered nickel which has beenmixed with a gas-forming powder and pressed into a grid or perforated plate. The processforms a plate which is 75% porous. Active materials are introduced into these voids.

charger

Figure 1J Sealed nickel-cadmium cell

Batteries and emergency sysmrrti /

Sealed nickel-cadmium batteries

Gassing occurs as a conventional battery approaches full charge, and increases during anyovercharge due to electrolysis of water in the electrolyte by the current supplied but no •longer being used in charging. The gas is released through the vent to prevent pressure ;build-up and this loss, together with loss from evaporation, makes topping up necessary.While on charge, the active material of the plates is being changed, but when the change iscomplete and no further convertible material remains, the electrical charging energy startsto break down the electrolyte. Oxygen is evolved at the positive plates and hydrogen at the;negative.

Sealed batteries (Figure 1.3) are designed to be maintenance-free and, althoughdeveloped from and having a similar chemical reaction to the open type, will not lose waterthrough gassing or evaporation. The seal stops loss by evaporation and gassing isinhibited by modification of the plates.

Sealed cells are made with surplus cadmium hydroxide in the negative plate so that it isonly partially charged when the positive plate is fully charged. Oxygen is produced by thecharging current at the positive plate (4OH~-»2H2O-l-4e~ +O2) but no hydrogen isgenerated at the negative plate because some active material remains available forconversion. Further, the oxygen from the positive side is reduced with water at thenegative plate (O2+4e~ + 2H2O->4OH~), so replacing the hydroxyl ions used in theprevious action. The process leaves the electrolyte quantity unaffected. The hydroxyl ions,acting as current carriers within the cell, travel to the positive electrode.

Sealed batteries will accept overcharge at a limited rate indefinitely without pressurerise. Charging equipment is therefore matched for continuous charging at low current, orfast charging is used with automatic cut-out to prevent excessive rise of pressure andtemperature. Rise of pressure, temperature and voltage all occur as batteries reach theovercharge area, but the last two are most used as signals to terminate the full charge.

Battery charging

Charging from d.c. mains

The circuit for charging from d.c. mains includes a resistance connected in series, to reducethe current flow from th^e higher mains voltage. A simple charging circuit is shown inFigure 1.4. Feedback from the battery on charge isprevented, at mains failure, by the relay(which is de-energised) and spring, arranged to automatically disconnect the battery. Thecontacts are spring operated; gravity opening is not acceptable for marine installations.

Charging from a.c. mains

Mains a.c. voltage is reduced by transformer to a suitable value and then rectified to give adirect current for charging. The supply current may be taken from the 230 volt section andchanged to say 30 volts for charging 24 volt batteries. Various transformer/rectifier circuitsa're described in Chapter 2 and any of these could be used (i.e. a single diode and half-waverectification, two or four diodes and full-wave rectification, or a three-phase six diodecircuit). Smoothing is not essential for battery charging but would be incorporated forpower supplies to low-pressure d.c. systems with standby batteries, and for systems withbatteries on float.


relay

t -dc.

supply

1

^Jr

spring—

—^

?11

11

IbattMry11

O J*l/\/S/V/S' 'resistance

^-•»•••

|

MM

Figure 1.4 Battery charging from a direct current supply

The circuit shown (Figure 1.5) has a transformer and bridge of four diodes with aresistance to limit current. The resistance is built into the transformer secondary by manymanufacturers. Voltage is dropped in the transformer and then applied to the diodeswhich act as electrical non-return valves. Each clockwise wave of current will travel to thebatteries through D, and return through D2 (being blocked by the other diodes). Eachanti-clockwise wave will pass through D3 and back through D4. Thus only current in onedirection will reach the batteries.

transformer rectifier

mainsac

supply

resistanceice $

batteryoanery_ i _J

Figure 13 Battery charging from alternating current

Standby emergency batteries

Emergency power or temporary emergency power can be provided by automaticconnection of a battery at loss of main power. A simple arrangement is shown (Figure 1.6)for lead-acid batteries. This type of secondary cell loses charge gradually over a period oftime. Rate of loss is kept to a minimum by maintaining the cells in a clean and dry state, butit is necessary to make up the loss of charge: the system shown has a trickle charge.


emergencyload

L

^

trickle>charge> c

1 .

i dc. isupply

full|harge|

C ,

batteries

C

1,I'

>

t

AftU*:oi

rest

spri

1— — luu— rm

Figure 1.6 Emergency battery circuit

In normal circumstances the batteries are on standby with load switches (L) open andcharging switches (C) closed. This position of the switches is held by the electromagneticcoil against pressure of the spring. Loss of main power has the effect of de-energising thecoil so that the switches are changed by spring pressure moving the operating rod. Thebatteries are disconnected from the mains as switch C opens, and connected to theemergency load by closing of L.

Loss of charge is made up when the batteries are on standby, through the trickle chargewhich is adjusted to supply a continuous constant current. This is set so that it onlycompensates for losses which are not the result of external load. The current value (50 to100 milliamperes per 100 ampere hours of battery capacity) is arrived at by checking with atrial value that the battery is neither losing charge (hydrometer test) nor beingovercharged (gassing). : :

When batteries have been discharged on load the trickle current, ,setj6nly to make upleakage, is insufficient to recharge them. Full charge is restored by switching in the quickcharge. Afterwards batteries are put back on trickle charge.

10 Batteries and emergency system*

Battery installations and safety measures

The explosion risk in.battery compartments is lessened by (1) ensuring good ventilation sothat the hydrogen cannot accumulate, and (2) taking precautions to ensure that there is nosource of ipnition

Ventilation outlets are arranged at the lop of any battery compartment where the"lighter-lhan-air hydrogen tends to accumulate. If the vejt is other tnan direct to* theoutside, an exhaust fan is required, and in any case would be used for a large installation.The fan is in the airstream from the compartment and the blades must be of a materialwhich will not cause sparks from contact or electrostatic discharge. The motor must beoutside of the ventilation passage with seals to prevent entry of gas to its casing. Theexhaust fan must be independent of other ventilation systems. All outlet vent ducts are ofcorrosion-resistant material or protected by suitable paint.

Ventilation inlets should be below battery level. With these and all openings.consideration should bejgiven to weatnerproohne." ——— ——~~ The use 61 naked lights, and smoking, are prohibited in battery rooms and notices arerequired to this effect. The notices should be backed up by verbal warnings because thepresence of dangerous gas is not obvious. Gas risk is highest during charging or ifventilation is reduced.

When working on batteries there is always the risk of shorting connections and causingan arc by accidentally dropping metal tools across terminals. (Metal jugs are not used asdistilled water containers for this reason.) Cables must be of adequate size and connectionswell made.

Emergency switchboards are not placed in the battery space because of the risk ofarcing. The precaution is extended to include any non-safe electrical equipment, batterytesters, switches, fuses and cables other than those for the battery connections. Externallyfitted lights and cables are recommended, wi th illumination of the space through glassports in the sides or deckhead. Alternatively, flameproof light fittings are permitted.

Ideal temperature conditions are in the range from 15°C to 25 °C. Battery life isshortened by temperature rises_above SO °C, and capacity is reduced by low temperatures.

Emergency generator

There are a number of ways in which emergency power can be supplied. The arrangementshown in Figure 1.7 incorporates some common features.

The emergency switchboard has two sections, one operating at 440 volts and the otherat 220 volts. The 440 volt supply, under normal circumstances, is taken from the mainengineroom switchboard through a circuit breaker A. Loss of main power causes thisbreaker to be tripped and the supply is taken over directly by the emergency generatorwhen started, through breaker B. An interlock prevents simultaneous closure of bothbreakers.

A special feeder is sometimes fitted so that in a dead-ship situation the emergencygenerator can be connected to the main switchboard. This special condition breakerwould only be closed when the engineroom board was cleared of all load, i.e. alldistribution breakers were open. Selected machinery within the capacity of the emergencygenerator could then be operated to restore power, at which stage the special breakerwould be disconnected.

The essential services supplied from the 440 volt section of the emergency boarddepicted include the emergency bilge pump, the sprinkler pump and compressor, one of

Batteries and emergency systems \\

alarms / control systems

Figure 1.7 Emergency power supply

two steering gear circuits (the other being from the main board), and a 440/220 voltthree-phase transformer through which the other section is fed.

Circuits supplied from the 220 volt section include those for navigation equipment,radio communication and the transformed and rectified supplies to battery systems.Separate sets of batteries are fitted for temporary emergency power and. for a low-pressured.c. system. The former automatically supply emergency lights and other services notconnected to the low-pressure system. Batteries for the radio are not shown.


The switchboard and generator for emergency purposes are installed in one compart-ment which may be heated for ease of starting in cold conditions. The independent andapproved means of automatic starting (compressed air, batteries or hydraulic) shouldhave the capacity for repeated attempts, and a secondary arrangement such that furtherattempts can be made within the 30 minute temporary battery lifetime.

The emergency generator is provided with an adequate and independent supply of fuelwith a flash point of not less than 43 °C (110°F).

Emergency electrical power

In all passenger and cargo vessels a number of essential services must be able to bemaintained under emergency conditions. The requirements vary with type of ship andlength of voyage. Self-contained emergency sources of electrical power must be installed inpositions such that they are unlikely to be damaged or affected by any incident which hascaused the loss of main power. The emergency generator with its switchboard is thuslocated in a compartment which is outside of and away from main and auxiliarymachinery spaces, above the uppermost continuous deck and not forward of the collisionbulkhead. The same ruling applies to batteries, with the exception that accumulatorbatteries must not be fitted in the same space as any emergency switchboard.

An emergency source of power should be capable of operating with a list of up to 22^°and a trim of up to 10°. The compartment should be accessible from the open deck.

Passenger vessels

Emergency generators for passenger vessels are now required to be automatically startedand connected within 45 seconds. A set of automatically connected emergency batteries,capable of carrying certain essential items for 30 minutes, is also required. Alternatively,batteries are permitted as the main emergency source of power.

Regulations specify the supply of emergency power to essential services on passengerships for a period of up to 36 hours. A shorter period is allowed in vessels such as ferries.Some of the essential services may be operated by other than electrical means (such ashydraulically controlled watertight doors), others may have their own electrical power. Ifthe batteries are the only source of power they must supply the emergency load withoutrecharging or excessive voltage drop (12% limit) for the required length of time. Becausethe specified period is up to 36 hours, batteries are used normally as a temporary powersource with the emergency generator taking over essential supplies when it starts (Figure1.7).

Batteries are fitted to provide temporary or transitional power supply, emergencylights, navigation lights, watertight door circuits including alarms and indicators, andinternal communication systems. In addition they could supply fire detection and alarminstallations, manual fire alarms, fire door release gear, internal signals, ship's whistle anddaylight signalling lamp. But some of these will have their own power or take it from alow-pressure d.c. system. Sequential watertight door closure by transitional batteries isacceptable.

The emergency generator when started supplies essential services through its ownswitchboard, including the load taken initially by the transitional batteries. Additionally itwould provide power for the emergency bilge pump, fire pump, sprinkler pump, steeringgear and other items if they were fed through the emergency switchboard. •

Arrangements are required to enable lifts to be brought to deck level in an emergency.


Also, emergency lighting from transitional batteries is required in all alleyways, stairs,exits, boat stations (deck and overside), control stations (bridge, radio room, enginecontrol room etc.), machinery spaces and emergency machinery spaces.

Cargo vessels

tjtnergency power for cargo ships is provided by accumulator battery or generator.Battery systems are automatically connected upon loss of the main supply, and ininstallations where the generator is not started and connected within 15 secondsautomatically, are required as a transitional power source for at least 30 minutes.

Power available for emergencies must be sufficient to operate certain essential servicessimultaneously for up to 18 hours. These are: emergency lights, navigation lights, internal

—communication equipment, daylight signalling lamp, ship's whistle, fire "detection andalarm installations, manual fire alarms, other internal emergency signals, the emergencyfire pump, steering gear, navigation aids and other equipment. Some essential serviceshave their own power or are supplied from a low-pressure d.c. system.

Transitional batteries are required to supply for 30 minutes power for emergencylighting, general alarm, fire detection and alarm system, communication equipment andnavigation lights.

CHAPTER TWO

Electronic Equipment

An explanation of the operation of electronic equipment requires consideration of atomicstructure, and the part played by the outer valence electrons in the bonding of atoms and incurrent carrying.

The structure of an atom

The atom is familiarly depicted by a simple model showing a central nucleus whichcontains neutrons and positively charged protons, surrounded by orbiting negativeelectrons (Figure 2.1). The negative charge of each of the electrons is equal but opposite to

Copper

1 valenceelectrons

Aluminium

3 valenceelectrons

Phosphorus

5 valenceelectrons

Silicon

4 valenceelectrons

Figure 2.1 Simple atomic structure

Electronic equipment 15

the positive charge of each of the protons. Electrical balance of the individual atomrequires that the number of positive protons in the nucleus is matched by an equal numberof orbiting negative electrons. Loss or gain of electrons changes the atom to a positively ornegatively charged ion due to the alteration in this balance.

Valence electrons

Electrons orbiting at the greatest distance from the nucleus of an atom are termed valenceelectrons. They play a major role in the bonding together of atoms in materials orcompounds. Valence electrons are also able to detach themselves and take part in themovement of electrons associated with the flow of electrical current through an electricalconductor. In an insulator, the outer electrons are not easily detached.

Conductors and insulators

That metals are good conductors is explained by the large number of electrons which arepresent in the structure and available to take part in current flow. Insulators have a verystable atomic structure in which orbiting outer electrons are bound tightly to the nucleus.Good insulators (e.g. glass, rubber, various plastics) are all compounds, not elements.

Semi-conductor materials

The semi-conductor materials silicon and germanium are, in the pure state, elementswhich are neither good conductors like copper nor good insulators such as glass andrubber. Conductivity is related to the number of electrons freely available to take part incurrent flow, and the valence electrons in the semi-conductor materials are bound into thecrystal structure sufficiently tightly to make them poor conductors but not well enough tomake them insulators.

Atoms of silicon and germanium (Figure 2.2) have four valence electrons which takepart in covalent bonds within the crystal structure. There are alternative ways of depictingthe bonding arrangement (Figure 2.3) but only the important valance electrons are shown

' valence electrons

Figure 2.2 Covalent bonds in a semiconductor material

16 Electronic equipment

I I I I

I valence | | |electrons

I I I I

-O- -O- -O- -O —

I I I I .— O - — O — — O — — O —

. I I I IFigure 23 Alternative method of showing covalent bonds in a semi-conductor material

here. Each atom contributes one electron to the bond with another and also provides aspace or hole into which an electron from the other atom can fit. Having four electrons forbonding means that each atom is associated with four others in the covalent bonds. Atabsolute zero temperature all of the valence electrons in a pure crystal of semi-conductormaterial are firmly held, making the material an insulator. The effect of heat energy fromonly a moderate temperature is that thermal agitation will cause a few electrons to becomedetached and able to move about. Each leaves a hole in its system which tends to attractanother electron, so leaving another hole etc. The result of heat energy is the randommovement of a few freed electrons (negative charges) between holes, and equally, randommovement of the positions of the holes. Heat thus makes current carriers available and atambient temperatures semi-conductors will carry a very small leakage current if a voltageis applied. The number of current carriers released by thermal effect increases withtemperature, as does the current flow.

Thermistors

Conductivity of a semi-conductor material increases with temperature rise and falls astemperature drops. (This property is also found in carbon and the characteristic isopposite to that found in metals, whose resistances increase with rise in temperature.) Theexplanation for the negative temperature coefficient of resistance in silicon and germaniumis that valence electrons forming covalent bonds in the structure are displaced due toincrease of energy resulting from temperature rise. As the electron wanders it leaves a hole,and in effect two charge carriers are produced by break-up of an electron hole pair. At verylow temperature the structure is more stable.

The property of negative temperature coefficient of resistance is used in devices fortemperature measurement, thermal compensation and thermal relays for protectionagainst overheating. The devices are called thermistors. They may be produced frommaterials other than silicon or germanium because the properties of these are sensitive toimpurities. Preferred materials are mixtures of certain pure oxides such as those of nickel,

~ - -- Electronic-equipment 17

manganese and cobalt, which are sintered with a binding compound to produce a smallbead with a negative coefficient of resistance.

Thermistors with the opposite characteristic, of a rising resistance with increase oftemperature, are preferred for some applications. They are described as having a positivecoefficient of resistance.

P- and N-type semi-conductors

Semi-conductor materials such as silicon and germanium, in the pure state and at ambienttemperature, are able to pass only a minute leakage current when moderate voltage isapplied to them. They act as inferior insulators rather than conductors, because of the lackof current carriers. They can, however, be made to conduct freely by the addition of smalltraces of certain elements.

N-type semi-conductors

N(for negative)-type materials have enhanced conductivity as the result of the addition ofan impurity with five valence electrons instead of the four normal to semi-conductormaterials. Antimony, arsenic and phosphorus are elements which could be used to'donate' an extra free electron.

Impurity with 5 valence electrons

I I I I /

o— —o— — o— -o —

i i i i— o - -o- — o - - o -

I I I x II I I I

— o- — o - — o - - o -I x I I ' \

Figure 2A N-type material

The atoms of the impurity used are scattered through the semi-conductor material(Figure 2.4) with four of their valence electrons joining with the atoms of the parentmaterial in covalent bonds and the fifth left free in the structure to act as a current carrier.Full current flow in an n-type material is based on the movement of these electrons.


P-type semi-conductors

P-type semi-conductors are made by adding impurities such as aluminium, borium orindium. Each of these is able to produce the apparent effect of removing negative electrons,so leaving gaps or positive holes which transmit charge. The p-type impurities aresometimes termed 'acceptors' for this reason.

The explanation is that the additive atoms have only three valence electrons and canonly participate in covalent pair bonds with three, not four, of the surroundingsemi-conductor atoms (Figure 2.5). The hole available for the fourth valence electron is leftvacant, and because it represents the lack of a negative electron is considered theequivalent of a positive charge. Such a positive hole has an attraction for valence electronsof adjacent atoms, but when filled by one of these another hole is left. Random movementof the hole would result from high-energy electrons moving into the gap and leavinganother. Current flow through p-type semi-conductor material involves the movement ofholes from the positive terminal of the power supply towards the negative terminal.

Impurity with 3 valence electrons

I I I I

- o - - o - - o - - o -+0 i i *0

I I I I-o- - o - +o o - - o -

i I I l

- o+o -o- - o - "b o -

Figure 2.5 P-type material

Solid state devices

Various techniques can be used (Figure 2.6) to produce devices having two, three, four orfive layers which are alternately p and n, in a single homogeneous crystal or slice ofsemi-conductor material. Current is transferred through the layers by movement ofnegative electrons (as in metals) and of positive holes.

The term 'solid state' is used to describe these devices because the current carriers movethrough the solid material, unlike electrons in thermionic valves which move in space tocross the gap between cathode and anode, or current carrying ions moving betweenelectrodes in a liquid.


/ N typecrystal growth<

\_ P type

_ " liquid

N and P typeimpuritiesadded in turn

semi conductor

P ^t emitter

heatedIndium beads

doping by impurity

Figure 2.6 Two methods used to produce pn devices

basic N type — >•crystal

P ty

base

V^-^coliector

pe

Semi-conductor junction rectifier (two-layer device)

A semi-conductor junction rectifier is a wafer of silicon, germanium or other semi-conductor material which has been doped by the impurities mentioned above, or by othermaterials having similar effects, so that one part is p-type and the other is n-type (Figure2.7). A battery connected in circuit will cause positive holes in the p-section to be attractedtowards its negative terminal. Negative electrons in the n-section tend to move to thepositive of the battery. Both positive and negative carriers therefore move towards thejunction, when the battery is as shown in Figure 2.7a, and current is conducted in thecircuit and across between the two regions. If the battery is connected the other way roundthe effect is still that positive charges are attracted to the negative, and negative electronsto the positive terminals (Figure 2.7b), but no current flows because both carriers moveaway from the junction, leaving a gap in the circuit. The ability of the rectifier to switch offwhen reverse biased enables it to be used as the means of converting alternating to directcurrent.


junction

P type I n type

holes electrons

battery

B

gap4 »

battery reversed

Figure 2.7 Semi-conductor junction rectifier

Characteristic curves

Current-voltage characteristics for a semi-conductor junction rectifier are completelydifferent when voltage is applied in the forward direction compared with the effect ofreverse voltage (Figure 2.8). This is to be expected in a device that freely conducts whenforward biased and resists backward current flow. In order to obtain a reasonable curve,however, the scales are not related on either axis between forward and reverse parts.

A silicon rectifier shows a rise in current flow as forward voltage reaches about 0.5 volts.Very large current flow is produced by quite small further voltage increases.

Reverse voltage causes negligible 'leakage' current up to a value at which reversebreakdown occurs. This breakdown voltage (sometimes called zener or avalanchevoltage) varies from one diode to another. The leakage current depends on a few currentcarriers liberated by ambient temperature and because these are usually small in number,leakage current remains negligibly low and almost constant. Increase of ambient


30-4- Amps

Vo«t

Figure Z8 Characteristic curve for a semi-conductor junction rectifier

temperature will increase leakage current slightly by releasing more electron/hole carriers.The increased leakage from a small temperature rise is shown by the dotted line.

Majority and minority carriers

Majority carriers in p- and n-type semi-conductors are, respectively, holes and electrons,which are present as the result of doping with impurities. Main current flow in p-typematerials is associated with hole movement. In n-type materials the main current flow isinvolved with movement of electrons.

Minority carriers are those present in a doped semi-conductor material as the result offactors other than the doping. Thus leakage current in reverse biased diodes (see Figure2.8) occurs due to a few minority carriers present as the result of thermal energy breakingup electron/hole pairs.

In other solid state devices electrons may be emitted into p-type sections or holes inton-type sections, when they are termed minority carriers.

Barrier potential

A phenomenon frequently referred to in descriptions of activity at pn junctions is thepotential built up by diffusion of negative electrons from n to p regions and the appearanceof holes on the n side.


The p region gains a net negative charge as the electrons combine with holes, becausethe holes are in the covalent bond structure and not caused by loss of an electron from anindividual atom. On the other side of the barrier, the n region gains net positive charge asthe presence of the holes is due in effect to loss of electrons from impurity atoms which hadin themselves a balance between positive protons and negative electrons.

Very few free current carriers remain in the junction area as a result of the diffusion andit is sometimes for this reason called the 'depletion area'. Diffusion is limited as chargebuilds up due to repulsion of electrons by the net negative charge on the p side.

Reverse bias effectively widens the depletion layer. Forward bias narrows it andremoves the barrier.

Rectification

Direct current is considered by convention to flow from the positive terminal of a source ofsupply (whether generator or battery) around the circuit and back to the negative terminalof the power source. It flows continuously in one direction.

Alternating current changes its direction of flow in time with the frequency of thealternator producing it, and flows first in one direction around the circuit and then in theother, building up to a maximum and dying away. The form of build-up and decay followsa sine wave pattern and this can be shown by connecting a cathode ray oscilloscope acrossthe a.c. supply.

Where direct current is required from an a.c. installation to provide power for d.c.equipment, then a.c. has to be rectified. The process of rectification is one where the flow ofcurrent in one direction is permitted to pass, but flow in the reverse direction is resisted, orchannelled in the other direction.

The modern rectifying device is a semi-conductor junction rectifier. These are pn diodeswhich, like the thermionic valves and metal rectifiers formerly used, act as electricalnon-return valves when connected into an alternating current circuit. That is, they permitcurrent flow in one direction but resist a reverse flow.

Transformers in rectifier circuits

Transformers are included in rectifier circuits for battery systems to bring a.c. mainsvoltage down to the required level. Voltage of alternating supplies (but not direct current)can be increased or decreased with very small power loss. A transformer also improves thesafety of an installation by isolating mains from the equipment being supplied. (SeeChapter 4.)

Half-wave rectification

Figure 2.9 shows a transformed a.c. supply connected to .-. load with a rectifier (or electricalnon-return valve) in the circuit.

Referring to the secondary winding of the transformer, when terminal T is positiverelative to terminal B, conventional current flows in a direction that agrees with that of thearrow symbol representing the rectifying diode. Current passes through the rectifier to theload and the rectifier is said to be forward biased. When the situation changes and B ispositive relative to T, then current flow in the circuit would tend to be the other way. Thisflow is resisted by the rectifier.

The effect of the single rectifier is to produce half-wave rectification and, as with


Figure 2.9 Half-wave rectification

alternating current, this can be demonstrated using a cathode ray oscilloscope. The halfsine waves indicate unidirectional although not continuous flow of current through theload as a result of the pattern of voltage developed. To obtain a d.c. supply with less ripple,the pulsations can be reduced by a capacitor smoothing circuit.

Full-wave rectification

Both half-cycles of the alternating current input can be applied to the load with anarrangement of two diodes and a transformer having a centre tap (Figure 2.10). Each pndiode conducts in turn when the end of the secondary winding which supplies it has fullpotential relative to the centre tap.

Figure 2.10 Full-wave rectification with two diodes


A high-voltage transformer is needed for this method of full-wave rectification. Thedouble winding is more expensive than the cost of extra rectifiers for a bridge rectifier.

Bridge full-wave rectifier

Four pn diodes in a bridge circuit between the transformer secondary and the load willgive full-wave rectification without the need for a centre tap (Figure 2.11). Transformervoltage and size are smaller for the same output, and the diodes are exposed to half asmuch peak reverse voltage.

The diodes work in series pairs to complete a circuit carrying current through the load.When terminal T of the transformer secondary has higher potential than B, then currentfollows a path from T through diode D, to the load and completes its travel through D2

back to terminal B of the secondary. Current flows in the opposite direction whenpotential of B is higher than that of T. The path taken is then from B through D3 to theload and returning via D4 to terminal T. A unidirectional current flow is provided for theload and smoothing can be applied to reduce ripple.

Figure 2.11 Full-wave rectification with four diodes

Three-phase rectification

The one, two or four diode circuits described above, for the rectification of single phasealternating current, give a unidirectional output but the quality of the direct current ispoor because of the ripple. Smoothing of the d.c. is required for most purposes. For largepowers, the degree of smoothing would cause losses and add to the cost. Better results areobtained from a bridge of six diodes (Figure 2.12) as used for rectification of a three-phasesupply.


applied 3-phasea.c. voltage

d.c. output

d.c.

output

3- phasea.c.input

Figure 2.12 Three-phase rectifier

Zener Diodes (two-layer devices)

A semi-conductor junction rectifier connected in reverse will block current flow unless thevoltage is raised to the reverse breakdown level. This feature is shown in the section onthese devices, by the characteristic curves. Zener diodes are components which areconstructed in the same way but made with a specific reverse breakdown voltage, bycontrolling the manufacturing process. Breakdown voltage is governed by the material,amount of impurity added and thickness of layers. It can be varied from one to severalhundred volts.


The symbol for a zener diode is shown (Figure 2.13a) together with the reverse part ofthe characteristic. The curve shows leakage current produced by application of a smallreverse voltage and then massive current flow as the result of exceeding reverse breakdownvoltage.

The breakdown capability makes zeners useful as protective devices and in this role theyact as electrical relief valves. A zener connected in parallel with a sensitive meter can, byincorporation of suitable resistances into the circuit, be arranged to bypass excess currentresulting from increase of voltage. Zeners are able to perform a similar service inbattery-charging circuits and in shunt diode safety barriers for intrinsically safeequipment. The characteristic curve shows that after breakdown, voltage across the diode

. remains almost constant despite increased current flow. This feature makes zeners usefulas voltage references and voltage stabilisers. A simple voltage stabiliser (Figure 2.13b) hasa resistance in series with the diode to absorb voltage change and limit current flow as aprotective measure. The zener connected so as to be reverse biased will, provided that itoperates in the breakdown condition, accept changes in current flow while maintainingvoltage at the breakdown value.

Changes to the d.c. input voltage are absorbed by the resistor. The effect on the zener isthat its current flow will rise or drop, but zener and load voltage will remain constant at thebreakdown point.

cathode

anode

breakdownvoltage

symbol -I

4- resistanceJ- .A/WV

variablevoltage

Idc.)

B i

zenerdiode load

Figure 2.13 Zener diode characteristic and voltage stabilisation circuit


If load current increases, the zener current drops by the same amount. Fall in loadcurrent causes a rise in zener current. Again the diode acts as a ready bypass for changingcurrent without altering voltage, which remains at the breakdown figure.

Voltage stabilisers are fitted to maintain constant voltage from variable d.c. supplies.An example of their use is in battery systems where voltage drops during the dischargeperiod of the cells. Equipment operates more efficiently from a stabilised supply. Batteryvoltage must always be above that required by the system and above breakdown voltage ofthe zener. Sufficient cells must be incorporated for this.

Voltage doublet

The circuit shown (Figure 2.14) can be used to produce a voltage across the load which isalmost twice the peak value of the a.c. input, provided that the load current is small.Similar circuits are available to increase voltage by a factor of almost four, but only if loadcurrent is very small.

The voltage doubler has two semi-conductor junction rectifiers and two capacitorsconnected, in this version, in the form of a bridge. Alternating current from the a.c. inputwill flow clockwise and then anticlockwise, as shown by the arrows. Clockwise flow viarectifier R, will charge capacitor C, to approximately the peak of the positive voltagewave. Anticlockwise flow, also shown by arrows, through rectifier R2 witl charge capacitorC2 to approximately the peak of the negative voltage wave. The two capacitors are in serieswith each other and also with the load. They discharge to the load, and being in series theiropposing voltages add to give a doubling effect.

Voltage multipliers are used in television and radar equipment as an alternative to aheavier, larger and more expensive transformer and rectifier arrangement.

d.c. supply to load

•MAMAAMAA/*Figure 2.14 Voltage doubler

28 • Electronic equipment

Transistors

Rectifiers and zener diodes are two-layer semi-conductor devices. Transistors have threelayers which are arranged as either npn or pnp. Methods used to make transistors aresimilar to those for the manufacture of diodes. Their operation is based on the principlethat application of voltage will make negative current carriers move in one direction andpositive carriers in the other.

Operation of npn transistors

The simple sketch of an npn transistor (Figure 2.15) shows a battery A connected to theends marked collector and emitter. Another battery B is connected to the middle basesection and has a common connection with circuit A to the emitter.

Figure 2.15 Transistor with npn arrangement


With only battery A in circuit, no current passes through the transistor because junctionJ2 is reverse biased by the battery. That is, negative electrons are attracted to the batterypositive terminal and positive holes to the negative. Both sets of. current carriers moveaway from the junction and no interchange occurs to promote current flow. (Junction J, ispn forward biased relative to battery A and would pass current.)

When battery B is connected, it has the effect of causing current to flow across junctionJ[ because holes in the base are attracted to the negative terminal of B and electrons in theemitter layer are attracted to the positive. The electrons fill the holes and produce an excessof electrons in the p section base. The base is made thin and doped with a small amount ofimpurity to promote this effect.

Electrons emitted into the base by battery B (minority carriers) act as current carriersfor flo v from battery A. The base becomes temporarily n-type. Current flow from batteryA is g< verned by the number of carriers injected and this in turn depends on the strength ofthe voltage signal from B. If the strength of the small input signal from B is varied, theresult will be a change in current flowing from battery A and through the transistor. Use ofa transistor to control a large current with a small input signal is called amplification.

The symbol above Figure 2.15 for an npn transistor has an arrow on the emitterconnection pointing away from the vertical line representing the base. The emitter, baseand collector connections are also denoted by their initial letters but these are unnecessary.The base forms a T with the middle base connection; collector and emitter are at eitherside. The arrow points in the direction of conventional current flow.

Operation of a pnp transistor

The symbol for a pnp transistor (Figure 2.16) is identical to that of an npn type except thatthe arrow on the emitter shows conventional current flowing towards the base. Operationis explained in the same way but from the simple sketch it can be seen that the main currentcarriers are holes instead of electrons and the batteries are reversed.

With only battery A in circuit, current flow through the transistor is resisted by junctiori—Jj. Here, negative electrons in the base are attracted by the positive terminal of battery A

away from the junction; positive holes are pulled away by attraction to the negativebattery terminal. The junction is reverse biased.

When battery B is connected between the base and emitter, current flows through J,which is forward biased by B. Holes, as positive charge carriers, move towards the junctionunder the influence of the negative terminal of B. The few negative electrons in the base areaffected by the positive terminal of B and also move to junction J,. The mutual interactionis such that holes appear in the base (minority carriers) and these act as current carriers forflow from battery A. The size of current flow from A through the transistor is governed bythe strength of the input voltage signal from B. Variation of this will cause change incurrent flow through the device from battery A.

Amplification

In the descriptions of npn and pnp transistors, the same sort of circuit was used in eachwith small signal power from a side circuit B controlling the larger power in a circuit A.The transistor enables a signal, too weak in itself to be of direct use, to control a largerpower source (battery A in the examples). The control by a small available power over alarge usable power is called power gain or amplification. Transistors can be connected indifferent ways and they can be used for various purposes, including switching.

30

collector

base

o o oo o oo o oo o o

o o oo o oo o oo o o

emitter

J*n

J.

Figure 116 Transistor with pnp arrangement

Thyristors

There are a number of electronic devices which are classed as thyristors. The mostcommonly known are silicon controlled rectifiers (SCRs) and triacs. Like diodes andtransistors they have alternate p and n layers; but whereas diodes have two layers andtransistors three, silicon controlled rectifiers are four-layer devices and triacs have agreater number.

Thyristors are solid stale switches which are turned on by application of a low-levelsignal voltage through a trigger connection known as a gate electrode. A solid state orstatic switch has no moving parts to wear, or contacts which can be damaged by arcing.Electrical 'turn on' makes it ideal for remote operation and its small size makes it aconvenient component of control circuits. Despite their small size, thyristors can be used


to control currents greater than 1000 amps and voltages in excess of 1000 volts. They cantherefore replace large conventional switches.

Thyristors operate at a much faster rate than mechanical switches and some are used inservices where the switching rate is 25,000 times per second.

Silicon controlled rectifiers (SCRs)

Another name sometimes used for the silicon controlled rectifier is reverse blocking triodethyristor

SCRs are supplied for industrial use in a form which allows them to be easily connected.The type shown (Figure 2.17) has a pnpn pellet of semi-conductor material which ismounted on a short stud and protected by an enclosing metal or ceramic case. The stud isthe anode connection, the wire at the other end is the cathode. Circuits to be switched areconnected through the anode and cathode. The low-level signal (trigger) voltage is appliedat the third lead which is the gate electrode. The SCR with its three connections isrepresented by an arrow as for a semi-conductor junction rectifier with an extra lead forthe gate. It can be used as a rectifier with controlled operation or as a switch. Direction ofworking current is shown by the arrows.

anode

Figure 2.17 Silicon controlled rectifier (SCR)

SCR operation

An SCR in circuit will resist current flow in the working direction, apart from leakage(Figure 2.18), because the middle junction is np or reverse biased. Leakage is somewhatgreater than with a simple diode because the pn junctions on either side of the centre oneare sources of extra minority carriers when forward biased.

32 .Electronic equipment

reversebreakdown voltage

voltsholding current .

reverse blocking state

on state

forwardbreakdown range

volts

forward breakdownvoltage

mA

Figure 2.18 Characteristic curve for a silicon controlled rectifier

trigger circuit

alarm V77\ Isignal ̂ '

5.C.R. 1

iaccept

switch

powersupply

. o

Figure 2.19 An SCR used in an alarm circuit


normalgate signal

complete wave

moothing elfect n

\full current

choppedJwave

>\T: ; \reducedcurrent

I

r\! \

a.c.supply

silicon controlledrectifier

increased resistance reduces gatesignal strength to give late firing

load

silicon controlledrectifier

a.c.supply t

variable resistance capacitanceload

Figure 2.20 Simple circuits to give late firing of SCR and control over the level of rectified current


If applied voltage is increased, more mino r i t y carriers are emitted from the outerjunctions and leakage increases more t h a n in a diode. When external voltage reaches acertain level, carrier mul t ip l i ca t ion reaches avalanche proportions and complete break-down occurs. Once breakdown is b rought about , the previous high resistance disappearsand current flows wi th ve r> l i t t l e vo l t drop, as shown by the characteristic curve. The SCRwill remain switched on unless cu r ren t flow (and minor i ty carrier emission) drops below acertain holding value.

During normal operation the SCR is turned on by a gate signal or trigger with theanode/cathode voltage much lower than tha t required for breakdown. This workingvoltage could be anywhere along the range of breakdown voltages. Low working voltagerequires higher trigger current and vice-versa. Obviously working voltage will be muchless than breakdown voltage to avoid the risk of the device turning itself on.

The gate signal or trigger is needed for a t ime measured only in microseconds. Itsapplication is as to the base of an npn transis tor formed by the bottom three layers of thethyristor. This npn ' transistor ' includes the middle reverse biased junction which ittherefore switches on. The top junct ion is already forward biased and triggering of themiddle junction causes current flow to be initiated through the device.

Examples of use

The SCR is shown as a latching switch in an alarm circuit in Figure 2.19. The alarmcondition is used to apply the trigger and switch the alarm circuit on through the SCR.When the alarm acceptance but ton is pressed and released it momentarily allows currentto bypass the SCR which then switches itself off and breaks the alarm circuit.

As a controlled rectifier the SCR can be used for conversion ofa.c. tod.c.,but by varyingthe trigger timing part of the wave can be blocked and part let through (Figure 2.20).Smoothing of the whole or chopped wave will give higher or lower average current.Control of electric motor speed, for example in hand drills and for d.c. main propulsionmotors, is possible with thyristors. Control of excitation current in some alternators orcurrent in impressed current systems for hull protection are other uses.

Silicon controlled rectifiers are one-direction devices. Current flow in the reversedirection is blocked by two junctions; and in the forward direction by one (middle)junction which can be switched. Their use is mainly in d.c. power supplies.

Triacs are two-directional switches, designed for use in a.c. applications and able to beswitched to pass current in either direction. In effect, a triac works like two siliconcontrolled rectifiers connected back to back. Only one gate terminal is required withpositive or negative voltage for triggering in one direction or the other.

CHAPTER THREE

A.C. Generators

The operation of generators relies on the principle that whenever there is mutual cuttingbetween a conductor and a magnetic field, a voltage and resulting current will be inducedin the conductor. The flow of induced current is not random: it is governed by thedirections of cutting and of the field and can be found from Fleming's Right Hand Rule(Figure 3.1).

Magnitude of the induced voltage depends on the strength of the magnetic field, rate ofcutting and length of the conductor.

forefinger

thumb(motion)

Figure 3.1 Fleming's Right Hand Rule

36 A.c. generators

Simple alternator

The arrangement used in the majority of alternators to exploit the principle of generationis shown simply in the sketch (Figure 3.2). Mutual cutting between conductors andmagnetic fields is produced by rotating poles, the magnetic fields of which move throughfixed conductors.

The rotor shown has a pair of poles so that output is generated simultaneously in twoconductors. Reference to the Fleming Right Hand Rule will confirm the instantaneousdirection of conventional current indicated by the arrows. The two conductors (R and R,)are connected in series so that the voltages generated in them add together to delivercurrent to the switchboard. The rotating fields, although moving at constant speed, willcut the conductors at a changing rate because of the circular movement. Voltage inducedat any instant is proportional to the sine of the angle of the rotating vector. The pattern ofbuild-up and decline, and also reversal in the voltage induced, is shown by the sine wave R.

3-phase output

Figure 3.2 Simple practical alternator

A.c. generators 37

Voltage and current are generated in each of the pairs of conductors in turn - first in onedirection and then in the other - to produce three-phase alternating current. The effect inconductors Y and B is also shown.

Three-phase systems

Outputs from the three sets of conductors in the alternator are delivered to three separatebus-bars in the switchboard, This is necessary because of the voltage and current disparitybetween them at any instant.

Three-phase, four-wire systems use a single return wire which is connected to the neutralpoint of the star windings. Current jn the return wire is the sum of currents in theindividual phases. If loads on each phase are balanced with voltages equal and at 120°apart, the three currents will sum to zero and the return wire will carry no current. Thefourth (return) wire will carry a small current if there is imbalance.

Thjee-phase, three-wire systems have no return wire. This is acceptable for ships where,direct from the main switchboard, three-phase motors make up much of the load andunless there is a fault they take current equally from the phases. Also some imbalance isacceptable with a three-phase, three-wire system provided load is connected in delta.Supp ies for lighting, heating, single-phase motors and other loads are taken throughdelta- star or delta-delta transformers.

The neutral point

The majority of British ships use three-phase, three-wire distribution with the neutralpoints of alternators insulated (Figure 3.3). Very little current will flow through an earthfault on one phase, because there is no easy path for it back to the electrical system. Withsuch a system, an essential elecuvc motor with an earth fault can be kept running untilstoppage for repair is convenient. This would be as soon as possible to avoid a full-phasefault that would result if an earth occurred on another phase as well.

Although fault current is negligible with an insulated/unearthed neutral point,overvoltages are high. The transient likely is 2.5 x line voltage. Such a voltage surge iswithin the capability of the main insulation of marine electrical equipment which is testedto -2 x line voltage +1000 volts.

A few British vessels have electrical distribution systems with an earthed neutral (Figure3.4). This is a connection of the system, via the neutral point of the alternator, to the hullsteel. The result of not isolating the electrical system from the hull is that current flow from

.^an eanhfaulLorjLany phase has a path through the hull steel and earthed.neutral back tothe system. The availability of the path encourages higher fault current flow than is thecase where the neutral is insulated or connected by resistance. Equipment with an earthfault, where the system is earthed, must-be disconnected immediately if a fault develops.This can be effected automatically with an earthed neutral system because the level of faultcurrent is high enough to operate a trip.

Earth fault current is high with earthed neutral systems, but overvoltages duespecifically to earth faults are lower. The earthed system is chosen to limit overvoltagesand to give automatic earth fault location and disconnection.

Overvoltages due to switching are not affected by choice of earthing or insulating theneutral. These high surges, and the possibility of others from failure of the voltageregulator, mean that the same standard oj equipment insulation is required for botharrangements.

38 A.c. generators

bus-bars

aNernator (A)

earth fault

y\

Figure 3J Three-phase three-wire system with insulated neutral

bus-bars i

alternator

earthed neutral

Figure 3.4 Distribution with earthed neutral

earth lault

A--%|_t»|x-

1 motor

SI81V

I

A.c. generators 39

Stator construction

The stator is a tube made up of silicon iron laminations with axial slots along the insidesurface (Figure 3.5) in which the conductors are laid. The importance of the iron stator liesin its ability to strengthen the rotor magnetic fields which cut the conductors. Obviouslythe iron stator is also a conductor and will have, like the conductors, voltage and currentinduced in it by the rotating fields. It is to prevent circulation of unwelcome eddy currentsthat the stator is made up as a laminated structure of steel stampings.

For assembly, the slotted laminations (each insulated on one side) are built into a packwith a number of distance pieces. Substantial steel endplates welded to external axial barsserve to hold the laminations firmly. The distance pieces are inserted to provide radialventilation ducts for cooling air.

High conductivity copper in trie form of round wire, rectangular wire or flat bar is usedfor the conductors. Round or rectangular wire used in smaller machines is coiled intosemi-enclosed and insulated slots (Figure 3.6). This type of slot improves the magneticfield.

.•ndplat*

Figure 3.5 Siator construction

stator

wedge

slotinsulation

Figure 3.6 Cross-section of stator slot and winding

40 A.c. generators

Rectangular section copper bar conductors in large alternators are laid in open slots(Figure 3.7). The bars are insulated from each other and from the metal slot surfaces by amica-based paper and tape cladding. Wedges ate fitted to close the slots and retain thewindings. In some machines the wedges are of magnetic material which helps to make thefield more uniform, so reducing pulsation and losses. Bonded fabric wedges are used insome alternators. Slots may be skewed to reduce pulsation and waveform ripple.

Insulating materials used in the slots and around the conductors are porous. A methodof sealing is necessary to exclude moisture which would cause insulation breakdown. Thesealing procedure starts with drying the stator with its assembled-conductors. Thewindings and insulation are then impregnated with, synthetic resin or varnish andoven-cured.

The six-conductor winding in Figure 3.2 shows the principle of three-phase generation.The far greater number of windings in an actual machine require much more complicationin the winding.

wedge

pre-formed stator winding I hairpin type I

\ \V \ \ \ \ \ \ \ \ \ \ \ \ \ \A \

Figure 3.7 Bar-type conductors

Rotor details

An alternator rotor has one or more pairs of magnetic poles. Residual magnetism in theiron cores is boosted by flux from direct current in the windings around them and thiscurrent from the excitation system has to be adjusted to maintain constant output voltagethrough load changes.

With the exception of brushless alternators, the direct excitation current for the rotor issupplied through brushes and slip rings on the shaft. The copper-nickel alloy rings mustbe insulated from each other and from the shaft. They are shrunk on to a mica-insulatedhub which is keyed to the shaft. Brushes are of an appropriate graphite material andpressure is applied to them by springs. ;

A.c. generators 41

Cylindrical rotor construction

The cylindrical rotor (Figure 3.8) is constructed with axial slots to carry the winding,which forms a solenoid although not of the usual shape. Direct current from the excitationsystem produces a magnetic field in the winding and rotor so that N/S poles are formed onthe ai eas without slots. One rotation of the single pair of poles will induce one cycle ofoutpi t in the stator windings (conductors). An alternator with one pair of poles has torotate at 60 times per second to develop a frequency of 60 cycles per second. In terms ofrevolutions per minute, the alternator speed must be 60 x 60=3600r.p.m.

Figure 3JJ Cylindrical or turbo-alternator rotor

Projecting (salient) poles bolted to the periphery of a high-speed rotor would be subjectto severe stress as the result of centrifugal force. The effect is minimised by using thecylindrical type of construction with the poles being built into the rotor. Small diameter iscompensated for by length. . f

Alternators with one pair of rotor poles are designed for steam or gas turbine drivethrough reduction gears. For this reason they are sometimes referred to as turbo-alternators. Cylindrical rotors can also be wound with two pairs of poles.

Construction of the core is similar in principle to that of the stator. It is built up of steellaminations pressed together between clamping rings and keyed to the steel shaft.Ventilating ducts are formed by spaters. The semi-enclosed slots are lined with mica-basedinsulation and the insulated copper windings are retained by phosphor-bronze wedges.The end turns of the winding are held against centrifugal force by rings of insulated steelwire or other material. After winding, rotors are immersed in resin or varnish andoven-cured. -

42 /i.e. generatorsn

Salient pole rotor construction

Salient field poles are those which are secured to the periphery of the alternator rotor andtherefore project outwards. The word 'salient' describes only the physical construction ofthe rotor: it is not an electrical term. '

'Higher speeds

A rotational speed of 1800 r.p.m. in an alternator with two pairs of poles which is designedfor a supply-frequency of 60 Hz (cycles per second) produces severe stress as the result ofcentrifugal force. The solid poles are therefore keyed (Figure 3.9) or firmly bolted (Figure3'. 15) to flat machined faces on the hub. Rotor diameter is kept to a minimum and the mildsteei hub and shaft are forged in one piece. Coils are wound from copper strip interleavedwith insula t ing material. After manufacture! the coils are mounted between flanges of hardinsulating board on the micanite insulated poles.

polewindings

Figure 3.9 High-speed salient pole rotor

Lower speeds

Alternators designed for rotation at lower speeds with slower prime movers have a greaternumber of pairs of poles (Figure 3.10). One pair of poles induces only one complete cycle inthe stator windings per revolution, and where for example an engine-driven alternator isintended for operation at 600 r.p.m. it requires six pairs of poles. The number of pairs ofpoles, p, is found from proposed speed and system frequency

/(frequency)x 60

NoTp.m.)

Frequency,/, is in cycles per second or hertz (Hz), so the figure is multiplied by 60 to makeit cycles per minute in the calculation.

The laminated poles of multi-pole machines are riveted and strengthened for bolting totne rotor hub by a mild steel bar inserted in the steel laminations. The coils are of insulatedcopper strip wound on mica-insulated spools of galvanised steel. Spools with their

A.c. generators 43

Figure 3.10 Slow-speed salient pole rotor

windings are fitted onto the poles and the assembly is bolted to the machined surface onthe hub. Laminated poles have a damper winding which consists of copper bars in the polefaces, joined by sectionalised copper end rings.

Rotor diameter is larger on slow-speed machines to accommodate the large number ofpoles, but low rotational speed produces less stress from centrifugal force. The flywheeleffect is beneficial and this, together with careful balance, improves smooth running.

Excitation systems

The excitation system has both to supply and control the direct current for the rotor polwindings. Level of the excitation current and resulting pole field strength are automatcally adjusted by the voltage regulating component. Excitation of early alternators wiprovided by a small direct current generator; voltage control by a carbon pile regulateThis, often referred to as the conventional alternator, is described below together with t!vibrating contact, brushless and self-excited types. ;

44 A.c. generators

Carbon pile regulator

The alternator sketched in Figure 3.11 has a direct current exciter with its armaturemounted on an extension of the alternator shaft. The d.c. exciter is a shunt generator fromwhich the majority of the armature current is conveyed through brushes and slip-rings tothe alternator rotor windings. It provides the magnetic field which cuts the stator windingsas the rotor turns and induces in them a voltage and resulting current output. A small partof the current from the armature of the exciter passes to the shunt field to provideexcitation for the d.c. exciter itself.

Current flow through the exciter shunt field is controlled by a resistance made up ofcarbon discs packed into a ceramic tube. Resistance of the carbon discs (or pile) is variedby pressure change. The pressure is applied by springs on an iron 'armature' and relievedby the magnetic field of an electromagnetic coil. Current for the coil is supplied through atransformer and rectifier arrangement from the alternator output to the switchboard. Thisis designed so that variations in alternator voltage due to load changes will affect thestrength of the electromagnetic coil and alter the compression on and therefore resistanceof the carbon pile.

electro-magnet

Figure 3.11 Conventional alternator with DC exciter and carbon pile regulator

A.c. generators 45

The resistance of carbon is least when pressure exerted on it by the springs and armatureis greatest, and this occurs when low alternator output voltage causes the solenoid to beweak. With low resistance between armature and shunt field of the d.c. exciter, morecurrent flows to the shunt and the high excitation current produced is fed to the alternatorrotor and increases the voltage.

Resistance of the carbon pile is highest when pressure on it is reduced by a strong-solenoid field when alternator output voltage is high. A strong field pulls the iron armatureaway from the pile, against the pressure of the springs.

Springs shown in the sketch are of the coil type but those in a carbon pile regulator areleaf springs. The ceramic tubes and discs are fitted in a casing with cooling fins. Thesolenoid has an iron core which, like the ballast resistance, is set to give the correctcharacteristics. The trimming resistor and hand regulator are for adjustment of the initialsetting of the regulator.

Vibrating contact regulator

The operating coil of a vibrating contact regulator is similar to that of the carbon pile type.It is supplied with a transformed and rectified current from the alternator output (Figure3.12), and the field of the electromagnet is used to attract an iron armature against aspring. The spring acts as a voltage reference and the strength of the electromagnet isrelated to alternator output voltage. Increase of output voltage produces an increase instrength of the magnet, and the effect is that the plunger and lever are pulled up together

Figure 3.12 Alternator and DC exciter with vibrating contact A VR (greatly simplified)

46 A.c. generators

with the control contact. A drop in voltage and decrease of strength of the coil allows theplunger to be pulled downwards by the spring so that the control contact moves downalso. The position of the control contact is governed by alternator voltage through the coil.

While the control contact will move wi th voltage variation, the vibrating contact isvibrated continuously by a rotat ing cam at a rate of 120 times per second. The vibratingcontact touches the other during the upper part of each vibration and the control contact

-is free to be moved up by the other, being only in spring contact with the plunger level.High alternator output voltage, through the coil, causes the control contact to be lifted sothat the vibrating contact only touches briefly. Low output voltage and weaker field in thecoil allows the plunger to drop so that the contact time is longer., Closing of contacts has the effect of short-circuiting the resistance in series with the

shunt field of the d.c. exciter. Shunt current passes through the contacts in preference topassing through the resistor and the larger shunt current increases flux and output of theexciter. This in turn provides extra excitation and increases alternator output voltage.

Static automatic voltage regulator

The carbon pile regulator uses a magnetic coil powered from the alternator output.Strength of the field varies with alternator voltage and this strength is tested againstsprings which are the voltage reference. The moving contact regulator employs a similarmatching of alternator output effect through a magnetic coil against springs.

The availability of a small transformed, rectified and smoothed power supply from thealternator output makes possible the matching of it directly against an electronic referencein the static automatic voltage regulator. The direct current derived from the alternatoroutput is applied to a bridge (Figure 3.13) which has fixed resistances on two arms andvariable resistances (zener diode voltage references) on the other two. The zeners operatein the reverse breakdown mode, having been manufactured with a zener breakdownvoltage of very low value. As can be seen from the earlier description of zener diodes,voltage remains constant once breakdown has occurred despite change in current. Thisimplies, however, that changes in applied voltage, while not affecting voltage across thediode, will cause a change in resistance which permits change in current. As with a

switchboard

transformer rectifier

alternator

Figure 3.13 Stalic automatic voltage regulator (SAVR)

1 \J~L2y*smoothing -

zener I

«

j. i

• •

>

**— error, signal

L zener

A.c. generators 47

Wheatstone bridge, imbalance of the resistances changes the flow pattern and produces inthe voltage measuring bridge an error signal.

The error signal can be amplified and used to control alternator excitation in a numberof different ways. Thus it can control the firing angle of thyristors (Figure 3.14) through atriggering circuit to give the desired voltage in the brushless alternator described. It can beused in the statically excited alternator to correct small errors through a magneticamplifier arrangement. The error signal has also been amplified through transistors inseries, for excitation control.

switchboardrectifier

VOLTAGE voUaoeSENSNG comparator

alternatorexciter field

Figure 3.14 Error signal used to control thyristors in the excitation system

The brushless alternator

In this machine slip-rings and brushes are eliminated and excitation is provided not byconventional direct current exciter but by a small alternator. The a.c. exciter (Figure 3.15)has the unusual arrangement of three-phase output windings on the rotor and magneticpoles fixed in the casing. The casing pole coils are supplied with direct current from anautomatic voltage regulator of the type described in the previous section. Three-phase

48 A.c. generators

Figure 3.15 Brushless alternator

current generated in the windings on the exciter rotor passes through a rectifier assemblyon the shaft and then to the main alternator poles. No slip-rings are needed.

The silicon rectifiers fitted in a housing at the end of the shaft are accessible forreplacement and their rotation assists cooling. The six rectifiers give full-wave rectificationof the three-phase supply.

Static excitation system

Direct on-line started induction motors take six to eight times the normal full load currentas they are starting. A large motor therefore puts a heavy current demand on the a.c.system, causing voltage to dip and, where recovery from the dip is slow, also producing

A.c. generators 49

momentary dimming of lights and effects on other equipment. There is a limit to the size ofmotor that can be started direct on-line, but the ability of alternators to recover from largestarting currents has been enhanced by development of the static excitation system.

The direct current required fqr production of the rotor pole magnetic field is derivedfrom alternator output without'the necessity for a rotating exciter as described for thecarbon pile/d.c. exciter system or for the brushless machine.

The principle of the static or self-excitation system (Figure 3.16) is that a three-phasetransformer with two primaries, one in shunt and the other in series with alternatoroutput, feeds current from its secondary windings through a three-phase rectifier forexcitation of the main alternator rotor.

Excitation for the no-load condition is provided by the shunt-connected primary, whichis designed to give sufficient main rotor field current for normal alternator voltage at no

switchboard

seriesprimary

-^

X

s—>.

trim

reactor V shunt

•—A OOpflOO jj — L^pt fltfljiW J

-— * OPQPQO/1— ̂ M tOQ^O^- — —— — « r£ — I | ^"^

| ""^™

1 1 MTVW-V«WV

JJ-_ 1 °°1"1 sh^ t> f> f> series

trimming A.V.R.

capacitors- —^e*.L

•».T

secondary

secondary

pilotexciter

mainthree-phaserectifier

Figure 3.16 Static excitation system principle

50 A.c. generators

load. Reactor coils give an inductive effect so that current in the shunt winding lags mainoutput voltage by 90°. Build-up of voltage at starting is assisted by capacitors whichpromote a resonance condition with the reactors or by means of a pilot exciter (thesealternatives are shown on the sketch by broken lines).

Load current in the series primary coils contributes the additional input to theexcitation system to maintain voltage as load increases. Variations in load current directly

' alter excitation and rotor field strength to keep voltage approximately right.Both shunt and series inputs are added vectorially in the transformer. Diodes in the

three-phase rectifier change the alternating current to direct current which is smoothedand fed to the alternator rotor through slip-rings., Voltage control within close limits is achieved by trimming with a static AVR tocounteract small deviations due to internal effects and wandering from the idealload/voltage line. The AVR may be of the static type already described with the errorsignal amplified and fed to d.c. coils in the three-phase transformer. Changes in d.c. coilcurrent brought about by the AVR alter transformer, output enough to trim the voltage.

Transient volt dip and alternator response

A gradual change of alternator load over the range from no load to ful l load would allowthe automatic voltage regulator (AVR) and excitation systems described to maintainterminal voltage to within perhaps 2% of the nominal figure. The imposition of load,however, is not gradual, particularly when starting large direct on-line squirrel cageinduction motors. Starting current for these may be six times normal and their powerfactor very low, at-say 0.4% during starting. The pattern of volt dip and recovery when thesteady state of a machine running with normal voltage is interrupted by the impact load atthe starting of a direct on-line induction motor is shown in Figure 3.17.

The initial sharp dip in voltage followed by a slower fall to a minimum voltage is mainlythe result of the size and power factor of the load and reactance characteristics of thealternator. Recovery to normal voltage is dependent on the alternator, its excitationsystem and automatic voltage regulator; also the prime mover governor.

Both the 'conventional' alternator with d.c. exciter/carbon pile regulator combinationand the brushless machine described have error-operated AVR and excitation systems(Figure 3.18). The voltage has to change for the AVR to register the deviation from normal

voltage

steady state

period of change(transient)

steady state

effect of poor prime movergovernor response

Figure 3.17 Typical ro/r dip/recovery pattern for an alternator

10-1

-20. functional

•rror operated

Figure 3.18 Comparisons o/vo(l dip and recovery for different excitation systems

A.c. generators 51

fteo

52 A.c. generators

and to then adjust the excitation for correction. The suddenness of the initial volt dip(blamed specifically on transient reactance) is such that the response from the error-operated system cannot come until the dip is in the second slower stage. Thus neithermachine can prevent the rapid and vertical volt dip due to transient reactance, but thefaster acting voltage regulator of the brushless machine will arrest the voltage drop sooneron the slower secondary part of its descent. The carbon pile regulator is slow comparedwith the static type but better recovery by the brushless alternator is also achieved by fieldforcing, i.e. boosting the excitation to give a quicker build-up.

Static excitation systems make use of load current from the alternator to supply thatcomponent of excitation current needed to maintain voltage as load increases. Thiscomponent of excitation is a 'function', therefore, of the load. Field current is thus forcedto adjust rapidly as load changes. Voltage disturbances accompanying application orremoval of load are greatly reduced. Statically excited alternators have better recoveryfrom voltage disturbance and permit the use of large, direct, on-line starting inductionmotors.

Reference

Ball, R., and Stephens, G. W. (1982) 'Neutral earthing of marine electrical power systems'.Trans.l.Mar.E. 95, Paper 32.

CHAPTER FOUR

A.C. Switchboards and Distribution Systems

Deadfront switchboards are a safety requirement for a.c. voltages in excess of 55 V.Mechanical strength and a non-flammable construction are obtained with the use of sheetsteel for the cubicles, and a passageway of at least 0.6m is left at the rear for access.

Power factor

The power of a direct current system is found by multiplying together the readings ofvoltage and current (V x A = watts). The same procedure with an a.c. system will producea figure which is contradicted by the lower wattmeter readings of true power. The ratiobetween true and apparent power is termed the power factor.

true power watts kWPower factor= = =:

apparent power volt-amps kVA

The anomaly between d.c. and a.c. systems is the result of the continual change instrength and direction of alternating current. Build-up of current is accompanied by abuild-up of magnetic field which, as it cuts through the windings of alternators and motorsin the installation, generates a voltage which reacts against the main voltage. The effect ofthis inductive reactance is to cause the supply current to lag the main voltage. Lateness ofthe current means that part of it is useful and part is not. The useful part is registered on thewattmeter as true power, sometimes called the wattful component. The power factor figure(usually about 0.8) is also the cosine of the angle of lag of the current.

Only the true power output and losses have to be supplied by the prime mover.However, the sizes of alternators, motors and transformers are governed by the apparentpower.

Alternator circuit breakers

The air-break circuit breakers used for marine installations are frame-mounted andarranged for isolation from busbar and alternator input cable contacts by being movedhorizontally forward to a fixed position. The isolating plugs (Figure 4.1) are not designedfor making or breaking contact on load so the breaker must be open before the assembly iswithdrawn. Safety interlocks should be fitted to ensure that the circuit breaker assembly

54 A.c. switchboards and distribution systems

Figure 4.1 Air-break circuit breaker contacts

cannot be racked out or in with contacts closed. Maintenance is facilitated by the draw-outcompartments and extending guide rails which allow the breaker to be pulled outcompletely.

The alternator breaker for three-phase supply has a single unit for each phase, of similardesign to the example in Figure 4.1. The three units are linked together by an insulated barfor simultaneous operation. Main fixed and moving contacts are constructed ofhigh-conductivity copper, and as an aid to low contact resistance the faces are silverplated. Main contacts are designed to carry normal full load current without overheatingand overload current until tripped, when a fault occurs.

Interruption of current flow results in the production of an arc between contact faces.Arcing is severe with overload current but is not a serious problem during normaloperation. To prevent damage to main contacts, separate arcing contacts are fitted whichare designed to open after and close before main contacts. These supplementary contactsare of arc-resisting alloy such as silver tungsten and easily replaced if damaged. Theyshould be inspected after fault operation of the breaker. The arcing contact shown has aspring which pushes it forward to hold until after main contacts have opened.

Air-break circuit breakers with arcing contacts have always been used for d.c.switchboards but further development was necessary to improve arc control before suchbreakers could replace oil circuit breakers and be used for a.c. (pre-1940). Laterdevelopment has meant that they can be used on systems with voltages up to 3.3 kV.

Arc control requires that the arc be elongated and removed from the gap between thearcing contacts. Electromagnetic forces associated with the arc and thermal action cause itto move up the arc runners to the arc chute provided for the purpose. Thus the arc iselongated and finally chopped into sections and cooled by the splitter plates.

Arc chutes are of insulating and arc-resisting material. They confine the arc and produce

A.c. switchboards and distribution systems 55

a funnel effect which assists thermal action. Splitter plates are of metal (steel or copper) insome breakers, and in others of insulator material. Some breakers have horizontal rodsfitted to cool and split the arc Arc runners are fixed and not of the moving type in somedesigns.

Interruption of the arc is assisted by the current dropping to zero during the cycle(however, with three-phases the zero points in each phase are staggered). Contact openingis therefore followed by a current zero and this means that for the next part of the cycle, anarc has to be struck across a gap. Successful removal of ionised gas (from the arc whichresulted from contact opening) will increase resistance in the air gap between contacts.When gas remains, it provides a path across which the arc can re-strike. The rate at whichthe gas is removed is such that the arc will not re-strike more than two or three times.

Breaking speed is made as high as possible by powerful throw-off springs and lightconstruction of the moving arm assembly. Rebound at the end of the opening movement isprevented by anti-bounce devices.

Rapid closing of the breaker also helps to prevent damage and most are power, ratherthan manually, closed. Power is provided by a solenoid or by a spring which isautomatically rewound by a motor and left ready after each closing operation. Wheresprings are used, an emergency hand-tensioning method is arranged for use with a deadboard, so that the spring can be wound up ready for closing the breaker. After operation ofa spring-activated breaker, the rewind motor can usually be heard charging the spring forthe next time.

Alternator and system protection

Protective devices are built into main alternator breakers to safeguard both the individualalternator and the distribution system against certain faults. Overcurrent protection is byrelays which cut power supplies to non-essential services on a preferential basis, as well asbreaker overload current trips and instantaneous short current tripping. A reverse powertrip is fitted where alternators are intended for parallel operation (in some vessels they arenot), unless equivalent protection is provided by other means. Parallel operation ofalternators also requires an under-voltage release for the breaker.

Overcurrent protection

The alternator breaker has an overcurrent trip, but a major consideration is that thesupply of power to the switchboard must be maintained if possible. The breaker thereforeis arranged to be tripped instantly only in the event of high overcurrent such as thatassociated with short circuit. When overcurrent is not so high, a delay with inverse timecharacteristic allows an interval before the breaker is opened. During this time theoverload may be cleared.

Overload of an alternator may be due to increased switchboard load or to a serious faultcausing high current flow. Straight overload (apart from the brief overload due to startingof motors) is reduced by the preference trips which are designed to shed non-essentialswitchboard load. Preference trips are operated by relays set at about 110% of normal fullload. They open the breakers feeding ventilation fans, air conditioning equipment etc. Thenon-essential items are disconnected at timed intervals, so reducing alternator load. Aserious fault on the distribution side of the switchboard should cause the appropriatesupply breaker to open, or fuse to operate, due to overcurrent. Disconnection of faultyequipment will reduce alternator overload.


Inverse definite minimum time (IDMT) relay

Accurate inverse time ciJay characteristics are provided by an induction type relay withconstruction similar to that of a domestic wattmeter or reverse power relay.

Current in the main winding (Figure 4.2) is obtained through a current transformerfrom the alternator input to the switchboard. (The main winding is tapped and the tapsbrought out to a plug bridge for selection of different settings.) Alternating current in themain winding on the centre leg of the upper laminated iron core produces a magnetic fieldthat in turn induces current in the closed winding. The magnetic field associated with theclosed winding is displaced from the magnetic field of the main winding and the effect onthe aluminium disc is to produce changing eddy currents in it. A tendency for the disc torotate is prevented by a helical restraining spring when normal current is flowing.Excessive current causes rotation against the spring and a moving contact on the spindlecomes in to bridge, after a half-turn, the two fixed contacts, so that the tripping circuit isclosed.

Speed of rotation of the disc through the half-turn depends on the degree of overcurrent.Resulting inverse time characteristics are such as shown in Figure 4.3. In many instances ofovercurrent, the IDMT will not reach the tripping position as the excess current will becleared by other means. The characteristic obtained by the relay is one with a definiteminimum time and this will not decrease regardless of the amount of overcurrent.Minimum time, however, can be adjusted by changing the starting position of the disc.

iniumdisc

helical spring

Figure 4.2 Overload relay


OVERCURRENT

DEFINITEMIN.TIME

Figure 43 Relay inverse time characteristics

Alternator breakers have instantaneous short-current trips in addition to IDMT (orother type) relays. In the event of very large overcurrent these rapidly trip the breaker out.Without an instant trip, high fault current would continue to flow for the duration of theminimum time mentioned above.

Reverse power protection

Alternators intended for parallel operation are required to have a protective device whichwill release the breaker and prevent motoring if a reversal of power occurs. Such a devicewould prevent damage to a prime mover which had shut down automatically due to a faultsuch as loss of oil pressure. Reversal of current flow cannot be detected with an alternatingsupply but power reversal can, and protection is provided by a reverse power relay, unlessan acceptable alternative protective device is fitted.

The reverse power relay is similar in construction to a household electricity supplymeter (Figure 4.4). The lightweight non-magnetic aluminium disc, mounted on a spindlewhich has low-friction bearings,'.: positioned in a gap between two electromagnets. Theupper electromagnet has a voltage coil connected through a transformer between onephase and an artificial neutral of the alternator output. The lower electromagnet has acurrent coil also supplied from the same phase through a transformer.

The voltage coil is designed to have high inductance so that current in the coil lagsvoltage by an angle approaching 90°. Magnetic field produced by the current similarly lagsthe voltage and also lags the magnetic field of the lower electromagnet. Both fields passthrough the aluminium disc and cause eddy currents.

The effect of the eddy currents is that a torque is produced in the disc. With normalpower flow, trip contacts on the disc spindle are open and the disc bears against a stop.When power reverses, the disc rotates in the other direction, away from the stop, and thecontacts are closed so that the breaker trip circuit is energised. A time delay of 5 secondsprever ts reverse power tripping due to surges at synchronising. Reverse power settings are2 to 6 'o for turbine prime movers and 8 to 15% for diesel engines.

58 ,4.c. switchboards and distribution systems

side view

Figure 4.4 Reverse power relay

Under-voltage protection

Closure by mistake of an alternator breaker when the machine is dead is prevented by anunder-voltage trip. This protective measure is fitted when alternators are arranged forparallel operation. Instantaneous operation of the trip is necessary to prevent closure ofthe breaker. However, an under-voltage trip also gives protection against loss of voltagewhile the machine is connected to the switchboard. Tripping in this case must be delayedfor discrimination purposes, so that if the volt drop is caused by a fault then time is allowedfor the appropriate fuse or breaker to operate and voltage to be recovered without loss ofthe power supply.

Synchroscope

The operation of synchronising an alternator before paralleling with another machinecould be carried out with the type of synchroscope shown in Figure 4.5. With its use, twophases of the incoming machine can be matched with the same two switchboard phases.

The synchroscope is a small motor with coils on the two poles connected across red andyellow phases of the incoming machine and the armature windings supplied from red andyellow switchboard bus-bars. The latter circuit incorporates a resistance and anmductance^coil in parallel. The inductance has the effect of delaying current flow throughitself by 90° relative to current in the resistance. The dual currents are fed via slip-rings tothe two armature windings and produce in them a rotating magnetic field.

A.c. switchboards and distribution svstems 59

Figure 4.5 Synchroscope

Polarity of the poles will alternate north/south with changes in red and yellow phases ofthe incoming machine, and the rotating field will react with the poles by tu rn ing the rotorclockwise or anticlockwise. Direction is dictated by whether the incoming machine isrunning too last or too slow. Normal procedure is to adjust alternator speed unti l it isrunning very slightly fast and the synchroscope pointer turning slowly clockwise. Thebreaker is closed just before the pointer reaches the twelve o'clock position, at which theincoming machine is in phase with the switchboard bus-bars.

Another type of synchroscope (Figure 4.6) also uses the principle of resistance andinductance connected in parallel across two alternator phases to give a 90° lag in currentflow. The result is that a magnetic field is produced in the coils A and B in turn, first in onedirection and then in the other. The pairs of iron sectors are magnetised by the coilsthrough the spindles which act as cores.

The spindle and iron sectors magnetised by coil A, which is supplied through theresistance, have a magnetic field in step with voltage and current of the incomingalternator. This is because pure resistance does not give current a lag, as does theinductance in the circuit for coil B which makes current (and magnetic field) lag voltage by90°. The iron sector pairs, spindles and coils A and B are separated by a non-magneticdistance piece.

The large fixed poles above and below the spindle are connected across two switchboardbus-bars (the same phases as those in the alternator supplying the spindle coils). When thefield of coil A (and the incoming machine) is in phase with the bus-bars, the sectorsmagnetised by A will be attracted - one to the top coil and the other to the bottom - so thatthe pointer is vertical. This occurs regularly with the pointer rotating clockwise when theincoming machine is running too fast; also when the machine is too slow and the pointerrevolving anticlockwise. Adjustment of incoming alternator speed to match the switch-


bus-bars

pointer

fixed pole

> pure CN pure'. resistance 3 Inductance

llillltransformer connection

incoming alternator

i Y

Figure 4.6 Synchroscope

board supply frequency results in slower movement of the pointer. Ideally the speedadjustment would achieve a coincidence of phase and speed with the pointer steady attwelve o'clock. In practice, the breaker is closed when the incoming machine is runningslightly fast (pointer turning slowly clockwise) and the pointer passing 'five to twelveo'clock'.

Emergency synchronising lamps

The possibility of failure of the synchroscope requires that there is a standby arrangement.A system of lights connected to the switchboard bus-bars and three-phase output of theincoming alternator, shown diagrammatically in Figure 4.7, may be used.

If each pair of lamps were across the same phase the lights would go on and off togetherwhen the incoming machine was out of phase with the switchboard and running machine.The alternators would be synchronised when all of the lights were out. Such an


switchboard

bus-bars

COOi

3 phases of

ArCX>|

incoming alternator

B-OO

R

Y

B

R

Y

B

Figure 4.7 Emergency synchronising lamps

arrangement is not as good as the one shown where only lamps A are connected across thesame phase. Pairs of lamps B and C are cross-connected. At the point when the incomingmachine is synchronised, lamp A will be unlit and lamps B and C will show equalbrightness. The lamps will give the appearance of clockwise rotation when the incomingmachine is running too fast and anticlockwise rotation when it is running too slow.

Pairs of lamps are wired in series because voltage difference between incomingalternator and switchboard varies between zero and twice normal voltage.

A.C. earth fault lamps

The sketch (Figure 4.8) shows the arrangement for earth fault indicator lamps on athree-phase a.c. system. Each lamp is connected between one phase and the commonneutral point. Closing of the test switch connects the neutral point to earth. An earth onone phase will cause the lamp for that phase to show a dull light or go out, depending onthe severity of the fault.

Each earth lamp and the resistance in series with it provides a path for current flow tothe neutral. An earth on one phase will, when the test switch is closed, allow current flowthrough an easier path than that through the lamp and resistance. The lamp is, therefore,shorted-out and will show a dull light or none at all.

Transformers

The two coils of a simple single-phase transformer are of insulated wire and wound on thesame laminated iron core. Often the windings are shown separated as in Figure 4.9 (top).


bus-bars ,

1(

J

!h;

Y

. R

h--:I.

k..l|

fuses

resistances

_Lr lh

Figure 4.8 A.C, earth fault lamps

Alternating current applied to one winding produces an alternating magnetic flux(strengthened by the iron core) which in cutting the second winding induces a voltage in it,also alternating. The flux, although established in the core by the a.c. source energising theprimary winding, also cuts this winding and induces a voltage in it almost equal to theapplied primary voltage. The same voltage is induced in each turn of both windings so thatvoltage is stepped up or down in proportion to the number of turns. The primary windingis always the one connected to the power source; the secondary to the load.

Winding terminals in three-phase transformers are marked with capital letters: A, A2,B, B2, C, C2 for the phases and N for a neutral on the high-voltage (h.v.) winding. Lowvoltage (l.v.) windings are distinguished by small letters a, a2, b, t>2, ci C2 f°r 'he phasesand n for a neutral. These letters are used regardless of whether the winding is a primary orsecondary.

Transformers are incorporated in battery chargers, instrument "connections in a.c.work, and power distribution systems. With their use voltage can be stepped up or downby a simple and efficient means without change of frequency. There is no direct linkbetween low-voltage secondary systems and a high-voltage primary. The isolation ofsections allows one to be earthed at the neutral and another to be insulated. Short-circuitcurrents are limited in the secondary. Safety of working supplies can be improved withtransformer step-down and isolation from the switchboard.

Three-phase transformers

Primary and secondary windings of three-phase transformers can be connected in star ordelta, and various combinations can be used to suit a particular application. Star windinghas the ad vantage of the n e u t r a l point which is avai lable for an ea r th connect inn if required


bmnated iron core

a.c. power source(primary)

HGRAMMATIC

higher or tower loadvoRage(secondary)

series-connected low voltage windings next to core

ACTUAL

N 7series -connected

nigh voltage windings outside

Figure 4.9 Simple single-phase transformer

or for a fourth wire. The delta winding is useful for unbalanced loading but has no neutralpoint.

A step-down transformer (Figure 4.10) of 440/230 V for general supplies, wounddelta-star, would permit earthing of the neutral point for the low-voltage supply with thehigher voltage system supplying essential machinery having an unearthed neutral.Earthing of the low-voltage neutral reduces over-voltages when a fault occurs and at thesame time causes sufficient fault current to operate protective devices. Essential machineryon the high voltage system with an insulated neutral can be allowed to run if necessary withan earth fault.

Essential machinery sometimes takes its power from high-voltage alternators through astep-down transformer. With the high voltage circuit earthed but the essential machineryrequiring to be unearthed, a star-delta transformer can be used.

Instrument transformers

The use of instrument transformers for indirect connection of relays, synchroscopes andmeasuring instruments means that they are isolated from high voltage and current in maincircuits and working with much lower values from transformer secondaries. Thus theinstruments are safer and require less insulation.

64 A.c, switchboards and distribution systems

DELTA STAR

440V B insulated neutraliiput ii mail vottage supply

b- 230Vauppty

Figure 4.10 Step-down three-phase transformer for general supplies

Voltmeters and voltage coils of relays or watt-meters are connected to potentialtransformers (Figure 4.11). The construction is similar to that of power transformerspreviously described.

Ammeters and current coils of relays or watt-meters are energised through currenttransformers, the primaries of which are connected in series (not shunt) with load current.

R Y Btransformer

transformer

voltmeter coi

both cols power meters etc.

ammeter col

Figure4.il Instrument transformer


References

Adams, E. M., and Ellwood, E. (1974) 'A 3.3kV electrical system for large container ships'Trans.I.Mar.E. 86, Series A Part 9.

de Jong, W., and Robinson, J. N. (1986) 'Generator failures caused by synchronising torques'.Trans.I.Mar.E. 99, Paper 8.

Hennessy, G., Mclver, J., Martin, B., and Rutherford, J. (1977) 'Commissioning and operationalexperience of 3.3kV electrical systems on Esso Petroleum Products tankers'. Trans.I.Mar.£.89.

Rush, H. (1982) 'Electrical design concepts and philosophy for an emergency and support vessel'.Trans.l.Mar.E. 94, Paper 28.

Savage, A. N. (1957) 'Developments in marine electrical installations with particular reference to a.c.supply'. Trans.I.Mar.E.

Savage, A. N. (1961) 'Details and operating data of recent a.c. installations'. Trans.I.Mar.E.Willcc-v, A. H. (1969) 'Protective equipment in marine electrical systems'. Trans.I.Mar.E.

CHAPTER FIVE

A.C. Motors

The majority of motors on ships with alternating current as the main electrical power aresquirrel-cage induction motors with direct on-line starting. With the changeover from theuse of direct to alternating current, these motors were a simple and robust replacement ford.c. motors. Compared with d.c. motors (which needed constant maintenance of startingcontacts, brushes and commutators; replacement of starting resistances and cleaning) theroutine work on an a.c. motor is negligible. Additionally, they are much safer, beingnon-sparking and having no resistances liable to overheat.

Squirrel-cage induction motors

Three-phase alternating current supplied to the stator windings of the motor when themotor is switched on produces a rotating magnetic field. The field cuts through copperbars in the stationary rotor and induces in them a current. Current flowing through barsand end rings (Figure 5.1) produces a magnetic field in each bar in turn, which in reactingwith the rotating field causes the rotor to turn. Motor speed builds up until it almost equalsthat of the rotating field.

Squirrel-cage rotor construction

Large squirrel-cage rotors have copper bar conductors brazed to copper end rings. Insmall motors, the bar conductors and end rings may be of aluminium, cast with fan bladeson the end rings (Figure 5.2). The bar conductors are arranged in the form of a 'treadmill'type cage (squirrel-cage), but this is not obvious because they are embedded insemi-enclosed slots.

The rotor core is built up of silicon steel stampings individually insulated to eliminateeddy currents and keyed to form a rigid assembly with the shaft. Slots are skewed forsmooth starting and quiet runn ing . The purpose of the iron core is to improve magneticfield strength, so the periphery of the rotor is machined accurately to give the smallestpossible gap between stator and rotor.

The conductors are a drive fit in the slots to prevent any movement and completed coreand cage construction has great s t rength. The conductor bars may be insulated to cut straylosses.

The mild steel shaft is amply proportioned so that it has the stiffness required to carry

A.c. motors 67

one setof coils //

staler

flux associatedwith conductorbars

small air gap so thatfield strength is preserved

Figure 5.1 Squirrel-cage induction motor operation

copper bars embedded inIron laminations

end* rings

Figure 5.2 Squirrel-cage rotor

68 A.c. motors

the heavy core and conductors. (The air gap beneath the rotor would be reduced if weightcaused the shaft to sag.) The ball or roller bearings which are usually fitted locate the shaftaccurately so that the air gap can be kept smaller than would be possible with sleevebearings.

Stator construction

Stator windings of an induction motor can be arranged in various ways so that a supply ofthree-phase alternating current will produce a rotating magnetic field. The method shownin Figure 5.3 has three sets of coils at different pitch circles with a 30% overlap. The outer

outer setmiddle set

Inner set

coll positions

Figure 5.3 One method of Stator winding arrangement

set are connected as in Figure 5.4 so that current flow between A, and A2 will producemagnetic fields simultaneously in the four coils but that the polarity of the top and bottomwill be the same and opposite to that of the other two. Current flow follows in coil sets Band C, and then the sequence is repeated with the direction of current reversed. The effect isto cause a rotating field.

Coil sets are usually connected in delta (Figure 5.5) and the standard identification iswith the lettering shown.

The coils are pre-formed from copper wire or strip with insulation and pressed into openslots, or they can be wound into semi-closed slots in the stator core. The stator is made up

A.c. motors 69

set* Band C ^ ^ >•r» »tmllarly- _^-. 7connected

Figure 5.4 Method of connection of the coils

method of connectingone set of cote

3 phase deltapermanentlyconnected

( usual in motors )

3-phase starpermanentlyconnected

3-phase windingswith ends brought out

for star/deltaconnection etc

Figure SS Method of supplying coils

70 A.c. motors

of steel laminations, stamped to shape with slots before being clamped together. Thelaminations and insulation between them, as in the rotor core, prevent passage of eddycurrents (from the generating effect of the rotating field).

Direct on-line start ing

The device for direct on-line starting consists essentially of three contacts, which are closedto connect the three-phase supply from the switchboard to the stator windings of themotor (shown on the left of Figure 5.6). However, rapid operation is beneficial and toachieve this a closing coil is used, which is energised from a low-voltage d.c. control circuit(shown on the right of Figure 5.6). Closing the isolating switch makes main power

low volt operating supply |

US?sformer

main

bus-bars

fuse

fuses

spring

closing coil

main contacts ^

, '

4

— ̂

safetytrips

r \

fi>

tH —I stop button

• n • •W^J start button

running light

Figure 5.6 Direct on-line starter circuits

A.c. motors 71

available to the main contacts and, via the transformer and rectifier, to the operatingcircuit.

When the start button is pushed the closing coil is energised, and as the main contactsclose so also does the contact (S). Release of the start button will not interrupt the closingcoil circuit because continuity is maintained through the retaining contact (S).

The main contacts are closed against a spring and de-energising of the closing coil byopening the control circuit with the top button, or operation of safety trips, will cause thecontacts to open and the motor to stop. The closing coil acts as a no-volts trip to preventinvoluntary restarting of the motor after a power loss (except for steering gear motors).

Disadvantages

Simple direct on-line start, squirrel-cage induction motors have three disadvantages: (1)high starting current, (2) low starting torque, and (3) single-speed operation (apart fromslight slip with increase of torque). Characteristics are shown in Figure 5.7.

startingcurrent

starting^torque

running

torque

running

\ I current

speed

Figve 5.7 Typical characteristic curves for a direct on-line start squirrel-cage motor

Low-current starting

Where the high current of direct on-line starting is unacceptable, the squirrel-cage motorcan be started by means which reduce voltage and current in the motor stator. Adisadvantage of low-current starting is that initial torque is also reduced.

Star-delta starting

The three sets of stator windings have end connections which are brought out to a starterbox. Changeover contacts in the starter enable the six ends to be star-connected for

72 A.c. motors

switchboard supply

STAfl contactsclosed to start <fopen to run

DELTA contactsopen to startclosed to run

1

—

y1

J11

fh—

)

(

<^

,,I

<

'>-

>/

<!i

\<

A2

A,

*

d

<

i

3

Ba

M9 E

§1 >B,

'

<

' \

(

3Ci

M>MCi

isolating switch

main contacts

1 safety trips

•f MOTOR

Figure 5.8 Star-delta starter (see also Figure 5.4)

starting (Figure 5.8) and then, as the rotor comes up to speed, to be reconnected in delta.Star starting has the effect of reducing the voltage per phase to 57.7% of the line voltage.

Starting current and torque are only a third of what they would be with direct on-linestarting. The low-current start is obtained at the expense of torque and star-delta motorscan only be used with light starting loads.

Automatic switching to the delta running condition is preferred to manual changeoverwhich may be made too soon or too slowly and cause a current surge. In the delta runningcondition, phase voltage is equal to line voltage and the motor behaves as a straightfor-ward squirrel-cage type.

Built-in interlocks or double-throw switches prevent star and delta contacts from beingclosed together. The starter is also designed so that star contacts have to be made before itis possible to change to the run position.

Auto-transformer starting

Conventional transformers have primary and secondary windings, but a single windingcan be used as both primary and secondary for changing voltage. Primary voltage of 440 Vapplied across a single coil (Figure 5.9) can be tapped at some point along its length and atthe end to give a secondary voltage with a value in proportion to the position of the tap.

' Msia»or-i O

B>(O(B

• aCfl V

5. •<o

-

1

A.c. motors 73

position of tap governs. .. .- i —i secondary voltagefull supply voltage -'

1reduced load voltage

I

74 A.c. motors

This type of t ransformer is used only in a l im i t ed number of applications because of therisk thai a fau l t could cause pr imary voltage to be applied to the secondary load. One suchuse is in au to- t ransformer s tar ters for squirrel-cage motors.

The auto- t ransformer s t a r t e r (Figure 5.9) consists of switches and three-phasetransformer of the single w ind ing type in circuit between the mains supply and the motor.For start ing, the three-phase supply is connected across the transformer and the motor

'receives a reduced voltage from the secondary tap. Reduced voltage gives lower currentflow in the stator at s tar t ing and less torque. When speed builds up the switches arechanged over to cut out the transformer and apply full mains voltage to the motor.

Starting voltage can be varied by changing the winding tap; i.e. stator voltage andtorque are adjustable.

Soft starting of induction motors

The mechanical contacts used in switchboxes for induction motors could be replaced bydevices of the thyris tor type (described in Chapter 2). These solid state controlled switcheswould eliminate arcing, burn ing and mechanical movement with its attendant wear. Theyare also capable of dealing with up to 1000 amps and 1000 volts.

Another benefit of the use of thyristors is that , because their switching can be controlled,they provide a means of controlling current flow to a motor during starting. Eachalternating current half-wave from the mains can be passed through the thyristor in full orin part, as dictated by early or late triggering of the device. Soft starting of inductionmotors based on the use of thyristors provides an alternative to star-delta andauto-transformer methods.

Improved starting torque

Starting torque of a simple squirrel-cage motor is low in relation to the maximum possibleoperating torque, as can be seen from the characteristic curve. Starting torque can beimproved by increasing resistance of the rotor conductors. However, high resistance in thecurrent path results in high starting torque but poor performance"at speed', unless theresistance can be reduced as the speed builds up. Induction motors with wound rotors anddouble squirrel-cage motors are designed to start on load with resistance but to run atnormal speed with the resistance removed or compensated for.

Wound rotor motor

The wound rotor induction motor has three-phase stator windings and a wound rotor inwhich current is 'induced' in the same way as in a squirrel-cage rotor. No external currentis supplied to the rotor.

The three sets of windings on the rotor (Figure 5.10) are connected together at one end,with the other ends brought out to three separate slip-rings. At starting, the three-phasecurrent applied to the stator from the mains creates a rotating magnetic field. Current flowinduced in the windings by the rotating field circulates through slip-rings, brushes andexternal resistances so tha t high s tar t ing torque is developed. As the motor runs up tospeed the resistances are decreased and finally the current is short-circuited by thecommon connecting wire. The rotor is wound only so that it can be connected in serieswith external resistances to give high starting torque. After the starting period, inducedcurrent flows around the rotor windings in the same way as it circulates through barconductors and end-rings of the squirrel-cage rotor. Interlocks ensure correct startingsequence.

A.c. motors 75

mains supply

start

Figure 5.10 Wound rotor motor

Double cage rotor

The other method of obtaining high torque for starting on-load requires that the motor befitted with a double cage rotor (Figure 5.11). The inner bars are of large cross section, lowresistance copper, but bars in the outer cage are of small size and have higher specificresistance. The direct on-line method is used for starting and the three-phase supply to thestator produces a rotating magnetic field which cuts both sets of conductor bars. Initiallycurrent flow is induced mainly in the high-resistance outer cage. The inner conductor bars,

outer cagehigh resistance

inner cage./I low resistance

Figure 5.11 Double squirrel-cage rotor

76 A.c. motors

Torque

total

\njl«!. c?9?

Speed

Figure 5.12 Torque characteristics of a double squirrel-cage motor

although having lower resistance, impede current flow, makiiuutjag the rotating field; thislag is due to high reactance developed as the result of the fast rate at which the magneticfield cuts the stationary and then slow-moving rotor. As speed increases and slip isreduced, the rate of cutting drops and reactance of the inner cage lessens. The inner cagethen becomes an easier current path and takes over as the means of producing torque(Figure 5.12). The characteristics of both high and low resistance cages combine to givegood starting and running torque.

The direct on-line double cage motor provides a method of starting on-load without thewindings, slip-rings and resistors of the wound rotor machine.

Speed variation

Single-speed operation of induction motors is suitable for most applications. Howeverthere are forms of speed control available and these include the reintroduction ofresistance in wound rotor motors, and the changing of the effective number of poles whichcan be used with any induction motor.

Resistance control of wound rotor motors

The resistances used for starting wound rotor motors can be used to reduce speed,provided that the motor is operating against a heavy load. Reintroduction of theresistances changes the characteristic and allows a slip of up to 25%.

Pole change motor

The stator winding of a pole change motor is designed so that by means of switchingarrangements the stator coils can be connected to give a different arrangement andeffective number of poles. The arrangement shown (Figure 5.13) could be used to give two

A.c. motors 77

staler windingdelta

3 phase a.c. supplyFigure 5.13 Pole change arrangement

deltaconnection

• star connection

fixed speeds with constant torque. In the low-speed version the stator poles are in seriesbut delta connected; for high speed, a star configuration of poles in parallel is used.

For low-speed running five contactors are open and the three joined by the dotted lineare closed. The three-phase supply through these poles is delivered to the stator windingsin series in the delta arrangement.

In the high-speed version the stator windings are in parallel within a star arrangement.For this the three contacts joined by the dotted line are open and the other five closed (thetwo on the right connect the star mid-point and current is supplied to the star tips throughthe three contacts at the bottom of the sketch).

Synchronous motor

Rotors in synchronous motors are supplied directly with current from an outside source,unlike squirrel-cage and wound type rotors in which current is induced by the statormagnetic field.

Synchronous motors are constructed in the same way as alternators, with three-phasestator windings and rotors with salient poles. Like alternators, they require a d.c. supply tothe rotor poles from an exciter via slip-rings. They are not self-starting. Connection of athree-phase input to the stator will produce a rotating magnetic field, but the effect on thepoles is of equal attraction and repulsion and rotor inertia will prevent movement in either

78 A.c. molars

direction. Only if the rotor is brought up to synchronous speed like an alternator can therotor poles and rotating magnetic field lock together. The machine runs then atsynchronous speed only. There is no slip as with induction motors.

A pony motor can be coupled to run a synchronous motor up to speed, then with theexcitation switched on the synchronous motor is synchronised and the driving induction(pony) motor shut down." Another method of running up a synchronous motor employs solid copper barspermanently embedded in the rotor pole tips and short-circuited by rings to make it atemporary induction motor. The synchronous machine is started direct on-line or by oneof the low-current starting methods (e.g. auto-transformer start) as an induction motor. Atmaximum speed the d.c. excitation is switched on for synchronisation.

Synchronous motors have been installed for pump and fan drives, also for mainpropulsion. The need for a starting arrangement makes them little used except for powerfactor improvement in some systems. Improved power factor can be obtained by increaseof d.c. excitation.

Single-phasing

The loss of current through one phase in a three-phase supply is described assingle-phasing. The open circuit in a phase, often from a blown fuse, faulty contact orbroken wire, will prevent a motor from starting but a running motor may continue tooperate with the fault.

A motor running with a single-phase fault will carry excess current in the remainingsupply cables (Figure 5.14). Motor windings will have unequal distribution of current. Thesingle winding A could have more than normal full load current with the motcr on halfload. Maximum motor loading could bring current in A to double or treble this figure.

2.4 X normal

open circuit

AT FULL LOAD

2.4 X normal

2.9 X normal

Figure 5.14 Single-phasing in delta-wound motor

A.c. motors 79

Current in the remaining two supply cables tends to rise in almost the same proportion asthat in A. Flow through windings B and C in series becomes higher than usual at full load.

Single-phasing in a running motor can be detected by overload devices in the supplylines or through the overheating.

A single-phased motor arranged for automatic starting will not self-start and ifoverloads fail to operate may remain 'on' with consequent overheating. Overheating in astalled or running motor will cause burnout of the overloaded coil.

It is possible for single-phasing in a lightly loaded motor to remain undetected byelectromagnetic trips on the supply lines which monitor only current. Improvedprotection is given by thermistors placed in the windings to measure thermal effects.

Motor protection

Protection of motors is required mainly to prevent overheating which can causedeterioration of winding insulation and burnout, if severe. Overheating as the result ofoverload, stalling, single-phasing or prolonged starting period can be detected by a rise inline current and by temperat.ure change. Overheating as the result of high ambienttemperature or poor cooling due to blocked air passages can only be detected bytemperature rise within the windings.

Overload protection is required for all motors of more than O.SkW although differentrules apply to steering gear motors and others essential to safety or propulsion. Aconventional electromagnetic overload trip must have a time delay dashpot (similar tothose for d.c. switchboards) to allow for high starting current in direct on-line startedinduction motors. Unfortunately, an electromagnetic overload trip can be reset quicklyand a motor restarted repeatedly with the result of excessively high winding temperature,unless a temperature trip is also provided.

Each of the three supply phases of the motor (Figure 5.15) is fitted with an overload

Figure 5.15 Electro-magnetic overload device

80 A.c. motors

trip

amplifierv thermistor

Figure 5.16 Thermistor protection of motor windings

relay. Such an arrangement should detect single-phasing, where the symptoms are of highcurrent in two supply lines only as well as straight overload. Operation of any of the relayswill close the circuit to energise the time-delayed trip.

The thermistor is a thermal device which can be used in conjunction with anelectromagnetic overload trip. One of these inserted in each of the three windings (Figure5.16) would detect overheating from any cause. Thermistors are available with either apositive or a negative characteristic (Figure 5:17). The former type are more definite inoperation because there is a very sharp rise in resistance at a particular temperature (asopposed to gradual drop in resistance of the other sort). Positive thermistors can beconnected simply in series and the very small current which passes through them normallyis cut off by the effect of overheating in any one of them. Cessation of the minute checkingcurrent is used as the signal to operate the motor trip.

Alternative methods of detecting overload current employ directly or indirectly heatedbi-metal strips (Figure 5.18). Excessive current in any of the supply cables will causedeflection of the bi-metal strip through temperature rise. Thickness of the strips is used todelay tripping when a motor starts. (A thick bi-metal strip takes longer to heat.)

Short-circuit protection is also a requirement for motors of over 0.5 kW. Fuses of thecartridge/high rupture capacity (HRC) design are employed to provide the necessary

•s&Temperature >

Figure 5.17 Characteristics of (A) positive and (B) negative type thermistors

A.c. motors 81

bl-m«ltl ttrlp

Figure 5.18 Overload trip using indirectly heated bimetal strip

rapid interruption of high fault current. Because short-circuit current may be high enoughto damage normal motor contacts, the fuses may be arranged to break first in the event ofshort circuit. The secondary function effuses is to provide back-up for the other protectivedevices.

Single-phase induction motors

Motors for washing machines and other domestic equipment operate from the single-phase low voltage supply. Motors in the machinery spaces are connected to thethree-phase higher voltage supply. A three-phase supply to a delta or star windingproduces a rotating magnetic field as described in the section on three-phase motors. Asingle-phase motor has two main windings in which the single-phase supply developsopposite polarities that alternate only. This effect is sufficient to keep the rotor turningonce it has started, but an extra pair of windings (Figure 5.19) with a lagging or leadingcurrent is necessary to start the motor initially. The current from a single-phase supply canbe caused to lag by incorporation of an induction coil or lead, with the use of a capacitor.

main runwindings

startwindings

(a)

Figure 5.19a Inductance coil starting for single-phase motor

82 A.c. motors

capacitance

cut-out switch

rnatn runwindings

start winding

startwindings

(b)

Figure 5.19b Capacitor-start motor

Induction coil

If an induction coil is used, it is connected in series with the extra starter windings and acut-out switch. At starting, current from the mains passes straight to the main windingsbut via the coil to the starter windings. The a.c. supply voltage produces in the coil anincreasing and then decreasing current, first in one direction and then in the other. Themagnetising effect of the current flow builds up a magnetic field. As the field builds, it alsocuts the coils and fulfils the generator rule. An e.m.f. that opposes supply voltage isgenerated in the coil and this slows down the rate at which supply current changes in thecoil.

The effect is to delay current flow to the extra starter windings. A rotating magnetic fieldis therefore produced in the two sets of windings, which acts on the rotor in the same wayas in a normal induction motor. The rotor turns and, as it runs up to speed, a device(centrifugal trip) cuts out the starting circuit with its induction coil and windings.

Capacitor starting uses a similar circuit.

CHAPTER SIX

D.C. Generators

Operation of a d.c. generator relies (as with alternators) on the principle that whenmagnetic lines of force are cut by a conductor (Figure 6.1) a voltage is induced in theconductor. Size of induced voltage and resulting current are dependent on magnetic fieldstrength, length of conductor and speed of cutting.

The direction of current flow is dictated by the relationship between magnetic field anddirection of movement of the conductor. It can be found from Fleming's Right Hand Rule,which is applied to give direction of conventional current flow during generation, alsoshown in Figure 6.1.

A simple generator can be constructed from a loop or coil of wire mounted on a spindleand arranged for rotation between opposite magnetic poles (Figure 6.2). The field-cutting

forefinger

' (field)

thumb(motion)

RIGHT HAND

Figure 6.1 Right Hand Rule of generation

84 D.c. generators

180 360

Figure 6.2 Simple generator

action of the straight sides will cause current flow as the result of induced voltage.Direction of flow is shown by the arrows (found from the Right Hand Rule) and can beseen to be continuous around the loop. The voltages generated are in series and thereforeadd to give twice the voltage produced in one side.

Direct current can be collected from the wire ends through the commutator whichconsists of two half-rings with brushes. Each brush takes current from one half-ring inturn, so that current flow is always in the same direction for each collecting brush. Theoutput is not steady but has a wave form.

Practical direct current generator

A practical direct current generator has a large number of conductors which are caused torotate in the magnetic field, not a single loop or coil as shown for the simple machine. Theconductors are coils, wound to shape on a former and insulated (Figure 6.3). They arefixed in the axial slots of a cylindrical rotor or armature and retained by wedges andbinding wire of tinnec steel (Figure 6.4). The coils are assembled in an overlaparrangement and coil span is equal to pole pitch. When the armature is turning, the twosides of several coils are simultaneously passing a pair of poles and having currentgenerated in them.

The magnetic poles of a d.c. generator (Figure 6.5) are magnetised iron cores withsuperimposed windings. The residual magnetism of the cores is essential to the initiation ofcurrent generation. When the generator is started, the rotating armature windings have

D.c. generators 85

Figure 6.3 Armature for a DC generator (or motor)

windings

_ - _ i - _ r - - - binding~

' V.) •, armature' - . • ' • ' • core

Figure 6.4 Detail of winding in slot

current induced in them only because they cut through the weak fields produced byresidual magnetism in the iron cores of the poles.

The small generated current passes to the windings on the poles via the commutator andbrushes. The electromagnetic effect greatly boosts the initially small residual field. Fieldpole windings as shown in Figure 6.5 can be connected in parallel with the armature and

magnetic

fietd

Figure 6.5 Arrangement of field poles and armature with its symbolic representation

load, as in the shunt generator (Figure 6.6), or in a series, as in the series generator (Figure6.7), or with a combination of both, a.s in the compound machine (Figure 6.8). The lastarrangement is most commonly used.

Shunt generator

The field pole windings of the shunt generator (Figure 6.6a) are permanently connected inparallel with the armature. Thus the small current produced in the armature conductorswhen they cut through the weak residual magnetic fields of the poles at start-up is delivereddirectly to the windings and serves to boost the magnetic field strength. Voltage builds upas the armature conductors cut through the strongr- fold, and in turn the higher voltage

D.c. generators 87

shuntfield

Figure 6.6* Shunt generator

load

VOtTAGE

CURRENT

Figure 6.6b Shunt generator characteristic voltage droop with load increase

pushes a greater current through the field windings. The mutual build-up causes voltage toreach its peak very quickly.

Voltage is restricted to a certain maximum by the use of many turns of fine wire in theshunt windings which gives high resistance and limits current flow through them. Furtherrestriction with control can be incorporated by fitting a variable resistor (or rheostat) onthe shunt field circuit.

A shunt field generator with no load will run with its maximum voltage. If the output isused for load, the terminal voltage will droop (Figure 6.6b). Decrease of terminal voltagewith steady increase of load current from zero can be plotted to show the typicalcharacteristic for a shunt generator.

The drooping voltage characteristic of shunt generators makes them unsuitable forsupplying current to equipment which requires constant voltage, unless a voltage

88 D.c. generators

regulator is fitted. Such a regulator would sense voltage variation (in the same way asthose described in the alternator section) and adjust the shunt field rheostat to correct thedeviation.

On the other hand, the drooping voltage characteristic of shunt generators gives themgood load-sharing capability. Increase in load current of one machine of a pair in parallelwould cause its voltage to droop so that the other higher voltage set would immediatelytake the load back.

Series generator

The field poles of a series generator are connected in series with the armature but thecircuit is only complete when the breaker is closed and there is a load (Figure 6.7a). Withno load, the machine relies on the residual magnetism of the pole cores for production ofterminal voltage. The weak field induces low voltage until load current augments fieldstrength, when the terminal voltage rises. Increase in current taken by the load results ingreater terminal voltage until the field reaches saturation point. The characteristic voltageand load current curve is shown in Figure 6.7b. Small initial e.m.f. is due to residualmagnetism.

no current

load

Figure 6.7a Series generator

VOLTAGE

CURRENT

Figure 6.7b Series generator characteristic

D.c. generators 89

Series windings are heavier than those for shunt fields, because they carry full loadcurrent and must have low reskmnce to keep volt drop small. Correct flux density isobtained with fewer turns of wire because of the higher current.

Serift generators have limited use for special applications.

Compound generator

The different windings and characteristics of shunt and series generators are combined inthe compound generator to give almost constant voltage over the operating range (Figure6.8c). Both shunt and series windings are mounted on the same pole cores but one set isconnected in parallel with the armature, the other in series.

Shunt windings consist of many turns of fine wire, as in the shunt generator. Heavy wireor strip used for the series windings is wrapped over the shunt coil and both sets aresuitably insulated and protected.

The shunt field provides full voltage after the initial build-up, at no load. Withincreasing load the shunt field voltage droops but the series field voltage rises and analmost level characteristic is obtained. The shunt, series and compound curves are shownin Figure 6.8c. The ratio between shunt and series windings can be altered to give over-,

seriesfield

\^

shunt field

Figure 6.3» Long shunt connected generator

seriesfield

shunt field

Figure &8b Short shunt connected generator

D.r. generators

VOLTAGE

CURRENT

Figure 6.8c Characteristic of compound generator, combining those of shuni anil .series types

VOLTAGE

I

CURRENT

Figure 6.9 Characteristics for a compound generator

D.c. generators 91

level- or under-compounding as shown in Figure 6.9. Slight under-compounding wouldgive good load-sharing as obtained with the shunt machine.

Discrepancy in load between two under-compounded sets would leave the machinewhich had the greater share of load with a lower voltage. The generator with the highervoltage would tend to take more load until the voltages were equal. However, generatorsmay be level or over-compounded so that equalising connections are used to stabiliseload-sharing.

The basic connection of shunt and series fields can be by long or short shunts (Figure6.8a,b). With the long shunt method, the shunt windings are in parallel with botharmature and series field. A short shunt is connected in parallel with the armature only. Alater diagram (Figure 6.16) shows a generator with shunt and series fields, shunt rheostat,interpoles and equalising connection.

Armature windings and the commutator

The simple commutator shown in Figure 6.2 is inadequate for a generator with a largenumber of overlapping windings which are simultaneously cutting through the fields ofseveral pairs of poles. The practical commutator is made up of a large number of coppersegments clamped to form a drum-like extension on the end of the armature (Figure 6.10).Each segment has connections from two adjacent conductor coils, as shown in the sketchof a lap-wound machine.

Figure 6.10 Commutator segment assembly

Lap-wound armature

The overlapping coils in Figure 6.11 are shown with one side as a full line and theo therasabroken line to represent coil sides in tops and bottoms of slots respectively. Coils A, B, C,D and E are, as the armature rotates, simultaneously cutting the magnetic fields associated

92 D.c. generators

coils

Figure 6.11 Lap winding

with south and north poles. The induced voltages cause current flow in the directions ofthe arrows, around the coils and through the coil ends and risers to the coppercommutator segments. The lap method of connecting the windings to the commutatorsegments puts all five windings in series between positive and negative brushes.

Carbon brushes are placed in contact with the commutator segment areas coincidingwith the range at which coils are in the gaps between magnetic pole fields. The brushes takepart in current flow with windings at both sides of their position, but at the same timemomentarily short-circuit the coils in the neutral range.

Coils moving past the gap have current induced in them in the opposite directionbecause of the change of magnetic field.

Overlapped coils are drawn separated for clarity in Figure 6.12.

Brushes

Brushes are housed in non-ferrous holders (Figure 6.13) in which they are free to slide.Contact with the commutator is maintained by spring-loading suitable to -the brushmaterial. Holders for a number of brushes are attached to arms and are adjustable. Sets of

D.c. generators 93

Figure 6.12 Windings separated to show current flow

tape loop

brush pressure adjustment

pressure arm

-spring balance

flexible securing screw

flexible

._ brush arm

thin paper betweenbrush and commutator

Figure 6.13 Brush and holder showing method of checking pressure (courtesy of NEI-APE-W.H.

94 D.c. gfnertilor.i

brushes fixed around the commutator for current collection can be staggered axially, sothat grooves are not worn on the surface. Circumferential position is also adjustable.

Brush material is basically carbon but it may be in the form of furnace processed orna tura l (mined) graphite. Apart from differences in the material, there are various gradesavailable. Choice of brush material and grade is governed by the intended duty.

Brushes on older generators and motors operate at low current density on low-speedmachines. Higher speeds and increases in commutated power required improvements forcool running and greater current capacity. Thus brush materials were developed withbetter thermal and electrical conductivity: they were made less hard, with improvedtoughness and reduced friction.

• Deposit of brush material and film build-up can result from brushes which are otherwisesatisfactory. One remedy has been to fit a complete set of brushes with cleaning propertieson one negative arm. Natural graphite brushes have good cleaning ability. Anotherproblem overcome by the use of special brushes is friction-induced chatter. There is arequirement that the brush position for satisfactory operation of a generator over the loadrange be marked, because of the problem of armature reaction.

incidentalarmature field

main field distortion

Figure 6.14 Magnetic field distortion (armature reaction)

D.c. generators 95

Armature reaction

Ideally, the magnetic field being cut by the armature conductor ends sharply at the edge ofthe poles and current flow in the armature conductors would also cease at the gap beforestarting again in the opposite direction. In practice, the main magnetic field is distorted byan incidental magnetic field resulting from current flow in the armature windings (Figure6.14). This 'armature reaction' means that current continues to be generated as coil sidespass into the physical gap between poles, and unless brushes are set further around on thecommutator they would be short-circuiting coils in which current was still beinggenerated. The effect would be to cause arcing between brushes and the commutatorsegments.

Interpoles and compensating windings

Unfortunately, the main flux distortion is greater with a strong incidental armature fieldand less when it is weak, so that changes of load current would require constantadjustment of brushes to a different neutral position. Automatic brush adjustment ispossible, but the neutral plane can be prevented from shifting by arranging an opposingeffect controlled by changes in armature current. Interpoles and comnensaiing windingsconnected in series with armature windings provide the solution.

Interpoles or commutating poles

The polarity of each interpole in a generator (Figure 6.15) is opposite to that of thepreceding main field pole and the same as that of the following pole, in the direction ofrotation. The interpoles are in series with the armature and, because they carry full loadcurrent, have heavy windings. Interpoles are represented in diagrams by the symbol usedin Figure 6.16. Flux provided by the few turns of interpole winding opposes the tendencyfor the field of the preceding opposite pole to be dragged around with the armature, soreducing the amount of armature reaction. Increases in armature current and fieldstrength which would further distort the main magnetic field are counteracted by a similarincrease in interpole flux, so that change of armature reaction is cancelled by the changingflux of the interpole. Over the load range of a generator the effect of the interpoles is toprevent change of armature reaction so that, with a stabilised neutral position, there is noneed for constant brush adjustment. However, brush position is always a little to one sideof the mid-point. Shaft magnetisation is avoided by arranging that current flows inopposite directions through main pole and interpole circuits. The currents and magneticeffects then tend to neutralise each other. This prevents generation of strong currents thatcould promote bearing damage.

Compensating windings

These are also connected in series with the armature so that current flow through them isthe same as, and varies with, that in the armature windings. They are fitted in slots in thepole faces parallel with the armature windings, but current flows through them in theopposite direction so that they-cancel the-change in armature reaction caused byvariations in armature current.

Compensating windings are used in machines having large load swings and consequent-ly massive changes in armature reaction such that rapid movement of the flux induces high

96 D.c. generators

interpoles

Figure 6.15 Position ofinterpoles between main poles

field

Figure 6.16 Connection ofinterpoles in a d.c. generator

D.c. generators 97

voltage in the armature windings. Without compensation, a sudden load increase couldcause a voltage rise in the armature windings sufficient to produce flashover betweencommutator segments.

Operation

Paralleling of a d.c. generator

The machine to be connected to the switchboard in parallel with another must, after initialstarting and a check of satisfactory running, have its voltage brought up to 220 volts. Therheos at on the shunt field circuit is used for this adjustment. The voltage of the runningmach ne is corrected if necessary and then the breaker of the incoming machine is closed.Finally, both generator rheostats are used to bring up the load current contribution of thenewly started machine and to reduce that of the other, slowly, until suitable sharing isachieved.

Machine fails to excite

Sometimes when a generator has been started, the voltmeter fails to register a reading inthe normal way. The pointer, instead of swinging up a little and then definitely movingalong the scale as the rheostat is turned, may remain at the zero position. It might show asmall negative reading. The most likely causes for this behaviour are the loss or reversal ofthe residual magnetism of the poles, respectively.

The residual magnetism of the poles is essential for the initial generation of the currentnecessary for the further build-up of shunt field strength. Correct direction of residualmagnetic field is also vital.

Both of these common faults are remedied by passing a current through the shunt fieldcoils, in the direction which will remagnetise the iron core in the right way. Current forrestoration can be obtained from another d.c. generator or from a battery. The operation

—is carried out with the faulty machine stopped.

Using another generator

The brushes of the faulty generator are taken out of contact with the commutatorsegments either by inserting a sheet of insulating material (thick chart paper) beneath themor by lifting them out of the holders. If the latter, they are rested on the holders or allowedto hang on the connecting wires. The rheostat is turned to zero so that there is fullresistance to current flow through the shunt field. The breaker for the machine which hasfailed to excite is closed while holding off the low-volts trip to allow this.

Closure of the breaker permits current from the running machine to pass to the faultymachine through the bus-bars. Backward flow of current will not pass through thearmatur-e or the-series field, because of the lifted brushes. Such flow through thelow-resistance armature and series field would cause damage and, being in the wrongdirection in the series field, would completely reverse the pole residual magnetism. Currentflow in the shunt field will be in the correct direction to re-establish residual magnetism.Current is in any case limited by the high resistance of the shunt winding (many turns offine wire) but is controlled by the rheostat. This is turned to the low-resistance position, sothat normal current passes through the shunt winding and the field is restored. The

98 D.i generators

rhcosiat is then turned back to ful l resistance before opening the breaker, so the breakerdoes not arc due to interrupting a current flow nor due to the stored energy in the coils.

After releasing the breaker, the brushes are replaced and the generator is restarted, whenresidual magnetism will initiate voltage generation; this should build up in the normalway.

Using a battery

Residual field of the pole cores can be restored with a 12 V battery which is connectedexclusively across the shunt field with the machine stopped. Current flow in the rightdirection for a few seconds only will establish the field.

CHAPTER SEVEN

D.C. Switchboards and Distribution Systems

The switchgear of d.c. switchboards, unlike that of enclosed a.c. equipment, was mountedon what was literally a board or screen of panels. On the front the open main circuitbreakers, distribution breakers, knife switches and instruments were easily accessible forinspection and maintenance. Safeguards against contact were only an insulated handrailand a rubber mat. A closed gate normally prevented entry to the passage"behind the boardwhere the copper bus-bars and rheostats were mounted.

Enclosed or dead-front boards are only required for d.c. installations if the voltage isgreater than 250 volts. Most of the d.c. switchboards on British vessels just prior to thegeneral change to a.c. were for 220 volt supplies.

Main circuit breaker

The brush-type moving contacts of the generator breaker (Figure 7.1) are closed against aresisting spring by a lift-and-latch handle. The handle, when raised, slips the operating pinover the latch, where it is clipped into position. Downward movement of the handle pushesthe cam plate against the hinged contact holder. The leverage and pressure applied forcethe brush contacts to spread on the fixed studs. This brushing action is beneficial in thatcontacts are wiped and any film or deposit is removed. Vibration with consequent burningor welding is prevented by the pressure, sustained by the resilience of copper strips, whichare clamped in the manner of a leaf spring. Conductivity of the copper is slightly reducedby a small amount of alloying material added to prevent work-hardening and to give thenecessary resilience.

The tripping of a main breaker due to overload or when load current was being carriedwould cause arcing and damage to the main contacts. Arcing contacts are therefore fittedand arranged to make first and break last, to protect main contacts from burning. They areof metallised carbon, sintered silver-tungsten or other material which does not weld athigh temperature; such compositions are-not^urtable for carrying high current over longperiods.

There is a set of main and arcing contacts for each connection to be made from thegenerator to the switchboard busbars. They are fixed together by bars for simultaneousmovement by one handle. Thus, for a two-wire insulated d.c. system the triple-pole circuitbreaker connects the positive, negative and equalising connections in one operation. The

100 D.c. switchboards anil distribution systems

Figure 7.1 Main circuit breaker for d.c. switchboard

equalising contact is given a lead so that it closes first. Short-circuit between poles isprevented by insulation.

The breaker is held in the closed position by the latching device but is easily opened bythe trip. Tripping is automatic in the event of short-circuit, overload, reverse current orlow-voltage faults. For normal opening, the trip can be operated manually after reducingthe load with the rheostat.

Generator protection

Generator circuit breakers are fitted with a number of protective devices. In general, formachines rated at over 50kW and intended for parallel operation, these are:

(1) an instantaneous short-circuit trip,(2) an overload trip with up to 15 seconds' time delay,(3) preference trips for shedding non-essential load,(4) a reverse-current trip,(5) an instantaneous under-voltage release (machine not generating)(6) an under-voltage trip with time delay.

101

On t.he d i s t r ibu t ion side the re are p ro tec t ive t r i p s and fuses which w i l l disconnectmotors or other c i rcu i t s in the even! of a local f a u l t . They also give protect ion to generatorsand, with their shorter time delays and lower cur ren t se t t ings , wi l l no rma l ly clear a faultbefore the main breaker trips. The term 'd i sc r imina t ion ' is used to describe the grading ofprotective device time delays and current se t t ings t h r o u g h o u t a d i s t r i bu t i on system.

Short-circuit, overload and preference trips are described below. They are all activatedby excess current and can be based on a coil connected in series to carry load current .

Instantaneous short-circuit t r ip

A simple electromagnetic relay (Figure 7.2) wi l l operate almost ins tan taneous ly when themagnetic effect is increased sufficiently by excess current to a t t ract the iron 'armature'against the resistance of the spring. The moving arm can open the breaker through a tr ipcircuit or by the direct movemen' ^f a mechanical l inkage .

The generator may be capable of supplying a shor t -c i rcui t current of up to twice the fullload current. Damage is prevented by setting the t r i p at much less t han th i s figure.

Overload trip

Various types of time delay are used for overload trips to ensure tha t the generator breakerwill not be opened due to a momentary current surge, but only if the excess currentpersists. Overheating and damage of machine windings and cable results from prolongedoverload current above the maximum rating.

Time lags of the dashpot type (Figure 7.3) delay t r ipp ing for up to a maximum of 15seconds when excess current attracts the plunger to the solenoid. The piston can onlymove up as the silicone fluid is displaced from top to bot tom th rough a small hole in the

spring

Figure 7.2 Instantaneous short-circuit trip

104 D.c. switchboards and distribution systems

armatura

Figure 7.5 Reverse current trip

superimposed field now weakens the left-hand field and, in strengthening the flux in theright-hand gap, pulls the armature anticlockwise to trip the breaker.

Under-voltage release/trip

Means to ensure that the main circuit breaker cannot be closed unless the generator isrunning and generating correctly can be incorporated into the under-voltage trip. Thelatter is intended to open the breaker at loss of excitation for any reason and has a timedelay.

The solenoid (Figure 7.6) is wired across the main leads on the generator side of thecircuit breaker and, when not energised, leaves the plunger in the trip position and thebreaker cannot be closed. Normal voltage output with the machine running energises thecoil and the plunger is pulled down against the loading spring to release the trip. Thisallows closure of the breaker and means that the device is set.

Parallel operation of d.c. generators (with equaliser)

The voltage of a d.c. shunt generator drops as its load current increases (see Figure 6.8).This characteristic makes shunt machines ideal for stable parallel operation because anyincrease of the load taken by one generator will reduce the voltage of that set, so that theother, higher-voltage generator will tend to take the load back. A machine taking less thanits proper share of the load will generate higher voltage and take load from the other.

Compound d.c. generators operating in parallel may be unstable because they lack thedrooping voltage characteristic. They are usually over-compounded, which gives them arising voltage with increase of load current. A machine taking more than its correct share

D.c. switchboards and distribution systems 105

breaker I

spring'

Figure 7.6 Under-voltage release

of the load would therefore generate higher voltage than before and actually increase itsload further.

Even identical machines have slight differences in speed or other characteristics whichwill cause one to develop slightly greater voltage than the other and to increase its load,although to begin with each has an equal share. The increased load current in onemachine, in passing through the series field of that machine, increases the field strength togive it higher voltage and load. Lower load current of the other machine weakens its seriesfield and reduces its voltage and load. Without some outside correcting factor the loadswing will continue until one generator has the full load and the other has none. To helpstabilise the parallel operation of over-compounded and level-compounded (which have apart-rising voltage characteristic) d.c. generators, an equaliser is incorporated.

The equaliser is a low-resistance circuit made between paralleled compound generatorsvia a special bar in the switchboard. It is shown by the dotted line in Figure 7.7, connectingthe armature ends of the series coils of the two generators. If the no. 2 generator develops aslightly higher voltage and starts to take more load, the greater current is not confined tothe series coil of machintno. 2. The equaliser passes part of the extra current to the seriescoil of machine no. 1. Field strengths of both machines are kept equal and a massive loadswing is prevented.

During the operation in parallel of compound-wound d.c. generators, there is acontinual hunting backward and forward of load as first one and then the other machinegene -ates slightly higher voltage. The equaliser limits the change of load by paralleling theserie . fields, so ensuring equal current and field strengths.

EC ualiser contacts in the main breaker are arranged to close before and open after themain contacts.

106 D.c. iwitcrthtmrilf and distribution systems

rheostat y

Figure 7.7 Generators in parallel with equalising connection

Prime movers

Load-sharing between d.c. generators is also stabilised if the prime movers, through theirgovernors, are given a slightly dropping characteristic. This means that as load increases,the governor undercorrects by a very small amount, and that over the load range from zeroto full load there is a speed loss of perhaps 5-6%. A machine taking more than its share ofload would run slower and the faster generator (with higher voltage from the speed) wouldtake it back.

Mechanical governors have a naturally drooping characteristic.

Earth lamps

Normal lights when supplied from a d.c. system can be connected across the full 220 voltsbetween positive and.negative leads. Earth lamps are small 30 watt lamps suitable for usewith a 220 V system,,but being a pair in series (Figure 7.8) the voltage effect is lowered andthe lights burn with reduced brightness. The mid-.point is permanently earthed by beingattached to the ship's steel structure.

D.c. switchboards and distribution systems 107

eartff - -.

Figure 7.8 Earth lamps for a d.c. switchboard

faiit 7

The main insulated two-wire d.c. distribution system is completely isolated electricallyfrom the ship's structure (unlike the formerly used single-wire system with hul l return),except for the mid-point earth between the earth lamps. A leakage of current throughfaulty insulation is made obvious by its effect on the lights because the fault provides analternative current path, bypassing the earth lamp on the fault side. A severe fault willprovide a path with much less resistance for current than the one through the earth lamp:the lamp will lose brightness or go out.

A typical failure is shown in the sketch, with insulation breakdown allowing a motorcasing to be 'live' and the fault current travelling to the ship's structure via the safety earthconnection on the motor casing.^The lamp or^the fault side Jin the example, the positive)darkens but the other one brightens. The brighter light is thus because it is connected on itsown across the full 220 V supply, albeit through an earth fault.

Another aspect is that with good insulation the positive bus-bar has a potential of+ 110V relative to the ship's structure and the negative -110 V, due to the mid-pointearth which must be at the same potential as the hull. A fault on one side puts that side tothe same potential as the hull and the other side 220 V different. The lamp connectedbetween the fault side and the mid-point earth has very little or no potential across it andgoes out. The lamp connected across the sound side and earth has the full 220 V and istherefore brighter than before.

The importance of earth lamps is that they give immediate indication of a currentleakage that is potentially dangerous. A number of fires have been the result of ignition bythe spark at an earth fault in wiring. The most dangerous condition occurs when there is afault on both positive and negative sides of the distribution at the same time. This amountsto a short-circuit across a 220 V supply, with the degree of current flow depending on theseverity of the faults. An additional problem is that equal-size faults will affect the lamps tothe same extent and slight dimness of both lamps may not be noticed. The check for such adouble problem is to switch off one lamp and look for an increase in brightness of theother.

CHAPTER EIGHT

D.C. Motors

Motor construction and operation

A d.c. motor is constructed in the same way as a d.c. generator, with inward projectingfield jioles in the yoke and a rotating armature witlLconductors in the slots (Figure 8.1).Input current is applied to the shunt and/or series windings on the field poles and also,through brushes and commutator, to the lap or wave wound armature conductors whichare momentarily passing under the poles. Current in the pole windings produces fixedmagnetic fields. Current in the armature conductors sets up a small magnetic field in eachof those under the poles. Reaction of the fields associated with the conductors and themagnetic fields of the poles produces a turning effect in accordance with the Motor Rule.This rule applies to all motors and can be summarised with reference to the simple sketch.

Current passing through a single conductor (Figure 8.2) sets up a circular flux and if theconductor is positioned in and at right angles to another field it will tend to be moved outof the field (as arrowed). Direction of movement of the conductor is dictated by themagnetic field and current flow in the conductor. Mutual directions can be worked outusing Fleming's Left Hand Motor Rule. With forefinger and second finger aligned with thefield and conventional currentflow respectively, the thumb points in the direction ofrelative motion of the conductor. Field direction is from north to south and conventionalcurrent flow is from the positive supply terminal and around the circuit to the negativesupply terminal.

The large number of conductors on a d.c. motor armature are connected in a lap orsimilar winding as in a d.c. generator. Current applied through the brushes andcommutator flows through conductors adjacent to the poles in such a way as to give anoverall turning effect in the same direction. For clockwise rotation of the motor shown(Figure 8.1), input current would flow into the page in the conductors under south polesand out of the page in those under the north poles.

Resistance starting of d.c. motors

Current flow from the mains supply through the armature conductors of a d.c. motorfulfils the motor requirement and causes rotation. However, rotating of the conductorsthrough the steady magnetic field of the poles also fulfils the generator condition. Avoltage termed back e.m.f. (electromotive force) is induced in the armature conductors.

D.c. motors 109

magnetk

field

Figure 8.1 D.c. motor with shunt and series (compound) windings

The back e.m.f. is in opposition to and a little less than supply voltage. The net effect, withsupply voltage being greater, is that current flows into the low-resistance armature as theresult of the small difference between the two voltages.

Back e.m.f. is not developed unless a motor is running, and the application of full supplyvoltape without an opposing voltage being generated in the motor would result in massivecurre: it flow through the armature. To protect the armature during the run-up from zeroto ful! speed, the starter has a resistance in series with the armature (Figure 8.3) to restrictcurrent. As motor speed and back e.m.f. build up, the resistance is cut out by movement ofthe starting handle.

Shunt motor starter

The starter shown in Figure 8.3 for a shunt motor illustrates the basic arrangement withthe return spring, hold-on (no volts) coil, and overload trip protective devices. The contactarm is in the off position and for starting is moved to the first stud contact with theisolating switch closed. This closes the circuit so that current flows through the startingresistances and armature (being restricted by the resistances). It also takes a path from thefirst stud through the shunt field via the hold-on coil. Application of current to the fieldpoles sets up a magnetic flux which rotates the armature because of the current in the

I 10 D.c. motors

i forefinger

conductormotion

^ fielddirection

Figure 8.2 Fleming's Left Hand Motor Rule

armature conductors, and in turn a back e.m.f. is generated in the windings as they cut themain magnetic field. This counter-voltage opposes the supply voltage and reduces itseffectiveness in pushing current through the armature.

As the motor speeds up, armature current drops until the contact arm is moved on to thenext stud, so cutting out some of the resistance and again increasing the current. A patternof current increase and reduction is produced as the contact arm is moved across andresistance cut out. At the final stud, the arm is held against the pull of the return spring bythe hold-on coil. In this position full mains voltage is available to the armature, but backe.m.f. limits the effective voltage and current flow. The starting operation takes a fewseconds, during which the contact arm is moved slowly but continuously from left to rightin the starter shown. Overheating of the resistance results if the arm is left at anintermediate position.

Shunt field current is taken from the first resistance stud, so that at the initiation of thestarting sequence full current passes through the shunt field. As the contact arm is movedto cut out resistance in the path for current flow to the armature during motor run-up, thesame resistances are left in the path for current flow through the contact arm to the shuntfield. The slight reduction in the small shunt field current and magnetic strength has theeffect of producing a moderate speed increase (see Field control, below).

Types of d.c. motor

The field poles of d.c. motors, l ike those of d.c. generators, can be connected with a shunt,series or compound arrangement and motors are classified by the type of connection.

D.c. motors 111

I resistances >• /*(\ AAA/Wl/!„ I

Figure 8J Resistance starter for a shunt motor

Some special-purpose motors are separately excited, and small motors may havepermanent magnet poles.

Shunt motors

The pole windings of a simple shunt motor are connected in parallel with the armature(Figure 8.4). Current input to the armature will vary but the field poles will take almostconstant current and as a result produce an almost unchanging magnetic field.

112 D-c. motors

shunt

field

d.c.supply

armature 1Figure 8.4 Shunt motor

A shunt motor used for driving a varying load will tend to slow down as the torqueincreases. Reduced speed means, that the rate at which the armature conductors cutthrough the pole flux is slower and back e.m.f. drops. Supply voltage remains constantand, with less back e.m.f., armature current will be increased. Torque of a shunt motorvaries directly with armature current, so drop in speed automatically produces greatertorque. Over the load range, speed drop is relatively small (about 10%, see Figure 8.5) andshunt motors are almost constant-speed machines. They could be used for variable loadand constant-speed drives (say centrifugal pump or motor generator drives).

field strength almost constant

Figure 8.5 Shunt motor characteristic

Series motors

A simple series motor (Figure 8.6) has field pole windings which are connected in serieswith the armature. Load current flowing consecutively through field and armature istherefore the same in each. Strength of the field will vary with armature current.

D.c. motors 113

Figure 8.6 Simple series motor

Armature current is governed by the difference between supply voltage and back e.m.f.A drop in back e.m.f. causes a larger margin of effective voltage and this in turn producesgreater current flow. Back e.m.f. in a series motor changes as speed varies with load, andalso because the magnetic field strength changes with load current. The speed torque curvefor a series motor (Figure 8.7) shows that, with low load, speed is excessive but the motorhas v;ry high starting torque. With high load, speed is low.

Th: series motor's characteristic of high starting torque would make it suitable for deckmach inery, but the dangerously high light-load speed has to be limited by a small shuntwinding in addition to the series field. A simple series motor requires a safety speed cut-outor permanently coupled load to prevent it from running away at loss of load.

SPEED

LOAD TORQUE

Figure 8.7 Series motor characteristic

114 D.c. motors

Compound motors

The poles of a compound d.c. motor have two sets of windings, one being in shunt and theother in series with the armature. There are alternative methods of connecting the fields.Figure 8.8 shows the 'short shunt', where the shunt field is connected in parallel with the

.armature only; Figure 8.9 shows the shunt field in parallel with both the armature andseries field, which is the 'long shunt' arrangement.

seriesfield

shuntfield

OUUL/- Tsupply

1Figure iW Compound motor short-shunt connected

shuntfield

Figure 8-9 Compound motor long-shunt connected

A cumulatively compounded motor is one in which both shunt and series coils areconnected so that total flux is produced by the sum of the individual fluxes. The ratiobetween the two sets of coils can be such as to give any characteristic between the twoextremes (Figure 8.10). A compound motor may be basically shunt-wound "with a fewseries windings on the poles for better starting torque, or it could be arranged with mainlyseries characteristics and with a small shunt field to limit light-load speed.

A differentially compounded motor is one with a weak series field connected so as tooppose and weaken the main shunt field when load increases. Reduction of field strengthcauses a motor to speed up, but in the case of a mainly shunt motor the series windings canbe designed to compensate for normal speed drop with load, so that the motor has almostconstant speed over the load range. Differential motors can be unstable.

D.c. motors 115

SPEED

LOAD TORQUE

Figure 8.10 Compounding characteristics

Speed control of d.c. motors

The effect of changing load on the simple motors previously described is that some degreeof speed variation occurs. Controlled speed change is imposed by using means to altercurrent through the field windings or armature. Conventional methods which employedvariable resistances or diverters have been replaced in modern d.c. motors by thyristorarmature control. The Ward-Leonard system is still used.

Field control

Change of current through field pole windings is easy to achieve by use of a rheostat(variable resistance) in series with a shunt motor field (Figure 8.11) or a diverter for thefield of a series motor (Figure 8.12). The weakening of the magnetic fields of the poles byreducing field current has the inverse effect of causing the motor to speed up. The

shunt field resistance

Tsupply

Figure 8.11 Shunt motor with field control of speed through a rheostat

116 D.c. motors

series field

AVAM—1

diverterI

supply

1Figure 8.12 Series motor with field control of speed using a diverter

explanation for this is that a weaker field causes less back e.m.f. to be generated in thearmature conductors as they cut the flux. The resulting larger difference between supplyvoltage and back e.m.f. means that there is more effective voltage to increase armaturecurrent. This method is useful for changing speed at the top end of the range, but it is notpossible to reduce speed below that which is obtained when full shunt-field current flows.

One limitation is that weakening of the magnetic fields of the poles permits them to bemore easily distorted by the rotating armature (armature reaction). Greater advantagecan be taken of field control if the machine has interpoles to limit the armature reaction.

Armature control

This can be used to give a speed range from zero to full with speed approximatelyproportional to armature voltage.

Rheostat controllers are variable resistors connected in series with the armature (Figure8.13) to alter the effectiveness of supply voltage across the armature. Speed changes as aconsequence. The method was used for d.c. deck machinery out of necessity but it waswasteful of energy. Apart from its inefficiency, there was the problem of resistances gettinghot due to the proportion of large armature current passing through them. The speedrange extends from zero to full.

armatureresistance

Tsupply

Figure 8.13 Shunt motor with armature speed control

D.c. motors 117

Thyristor control applied to the armature of a d.c. motor uses a rectifier arrangementthat includes silicon-controlled rectifiers. In the example (Figure 8.14) the armaturecurrent is taken through a three-phase rectifier which has an SCR and an ordinary rectifieron each section. Triggering by a gate signal (Chapter 2) which is early or late gives control.Current to the field pole windings is taken through a simple three-phase rectifier, andremains constant.

thyrlstor control

thyristors j [

a.c. supply

motorrect.

a.c. supply

three-phase rec t i f ie r

Figure 8.14 Thyristor control of d.c. motor speed

Ward-Leonard system

This system, used for fine control of d.c. electric motor speed from zero to full in eitherdirection, is also able to give the motor a robust torque characteristic. The system was usedfor the motors of electric (as opposed to hydraulic) steering gears of ships with d.c.electrical power, and it is used today on ships with a.c. electrical power for deck machinerysuch as the windlass.

The working motor (Figure 8.15) which powers the steering gear, windlass or otherequipment is a d.c. machine, because speed control of these is easy. The method is to alterthe voltage applied through the brushes to the armature windings of the d.c. motor; nochange is made to the current in the field windings. The voltage is increased or decreased,not with the use of resistances but by arranging an individual d.c. generator withcontrollable output voltage as the power supply for the armature of the working motor.Speed and direction of the working motor vary with the magnitude and direction ofapplif d voltage. Current for the windings of the field poles is derived from the source ofmain power. Where there is a.c. main power, current for the windings is transformed tolower voltage and rectified.

The d.c. generator is driven by a simple one-speed, direct on-line start, squirrel-cagemotor (a.c.-powered ship). Output of the generator is varied by changing the current to itsfield windings through a variable control resistor (potentiometer). As magnetic fieldstrength is altered by the change of field current, so too is the generated voltage. Switch ofdirection of current flow through the field poles, also with the potentiometer, will cause the

118 D.c. motors

mains B.C. Supply

transformer

.000000000

\

Figure 8.15 Ward-Leonard system

direction of the pole magnetic fields to change. This changes the direction of generatedcurrent supplied to the motor and thereby also the running direction of the motor.

The control lever can, by moving the potentiometer contacts in opposite directionsaway from the mid-position shown in the sketch, set the polarity and strength of thegenerator field poles. By governing generator output to the armature of the workingmotor, this in turn gives stepless speed control of the working motor, in either direction.

The system as used for steering gear operation on d.c. ships is complicated by thefeedback which automatically brings the potentiometer to its mid/neutral pos:tion, whenthe rudder gear reaches the desired position.

Maintenance of d.c. motors

The d.c. motor has the same construction and therefore the same general maintenancerequirements as the d.c. generator. Thus regular cleaning is needed to keep the air passagesclear and to remove deposits of dust, oil and grease which could reduce the effectiveness ofthe insulation. Readings are taken of insulation resistance at intervals. The commutatorand brushes are inspected and maintained as necessary to prevent sparking. Mechanicalchecks include bearings, holding-down bolts, and drive couplings or belts and pulleys.Motor starters and controllers require regular checks and maintenance of contacts,resistances and connections.

Before undertaking any work on electrical equipment, it must be isolated from the

_ . _ . . . _ . . . _ - - . D.c. motors 1 1 9

Electrical supply completely and a notice should be fixed on the isolator. Personnel should^e informed verbally and all circuit fuses removed and retained.

Contacts deteriorate in service as the result of burning and wear so that repair orr«newal becomes necessary. Damage can be rectified by dressing with a fine file to restorelhe profile accurately across the surface. Emery is used for the final polish. ContactsBecome thinner as the result of wear and cleaning, so that there is loss of contact pressure.loading is measured with a spring balance and corrected unless no adjustment is provided^hen contacts are renewed. Insufficient contact pressure results in welding together ofsUrfaces and failure to open under fault conditions.

A thin smear of vaseline or other contact lubricant is applied to some types of contactafter servicing. Light oil is used on pivot pins.

Resistors in motor controllers can be checked with a meter. Those of the cast grid type,sUpported at the ends, tend to crack due to vibration. Wound resistors fail through°verheating or general deterioration of the insulation. Resistor boxes must be wellvtntilated when motors are in use. Connections are examined from time to time forcbrrosion or for overheating due to slackness. Where necessary they are disconnected,cleaned up and remade.

CHAPTER NINE

Safe Electrical Equipment for Hazardous Areas

Electrical equipment and flammable atmospheres

Hydrocarbon gases or vapours from crude oil form highly flammable mixtures with air(Figure 9.1) when present in the proportion of between 1% and 10% hydrocarbon with99% down to 90% normal air. Below the lower explosive limit (LEL) the mixture is toolean to burn rapidly, although a lean mixture will bum slowly in the presence of a nakedflame or a spark, as is proved by the operation of explosimeters in this range. Over-richmixtures exist when the level of the hydrocarbon exceeds 10%.

The risk of explosion or fire within the cargo tanks, pumprooms and other enclosedareas of an oil tanker is high as a consequence of the very small quantity of hydrocarbongas or vapour that makes up an explosive mixture. Within tanks, vapour is readily evolvedfrom cargo: immediately above the surface of the liquid there tends to be a layer ofundiluted hydrocarbon vapour. Further above the surface of the liquid, dilution by air (ifpresent) increases towards the top of the tank, to give a mixture with a decreasinghydrocarbon vapour content. At the top of the tank, the larger quantity of air is likely toproduce a lean mixture. Thus the free space within tanks above any liquid petroleumcargo, or in an empty tank from which the cargo has been discharged, is likely to contain amiddle flammable layer sandwiched between lower over-rich and upper lean layers.Leakage of crude oil or products from pump glands would produce the same effect inpumprooms.

Inert gas is delivered to the cargo tanks of crude oil tankers to maintain a slightly higherthan atmospheric pressure and so exclude air during operations. The inert gas can also beused to displace any air present in a tank after inspection or other period when the IGsystem has not been in use.

The IG system cannot be used for pumprooms or other spaces in which crude oilvapours can collect and therefore form a flammable mixture with air. In these areas and ondeck, precautions to avoid the possibility of ignition are necessary.

There are no special measures taken in ordinary electrical equipment to encapsulatecontacts which arc as they close or open. Thus light switches, torches, sockets and plugs,pushbutton contacts, relays, electric bells, starters, circuit breakers and open fuses are allpotential sources of ignition should there be flammable gas, vapour or dust in theatmosphere. The danger from arcing contacts in a starter box or switch is not obviousbecause the arc is hidden. Sparks from an electric motor, particularly of the commutator

Safe electrical equipment for hazardous areas 123

Flameproof (Ex d) equipment

If electrical equipment could be made with perfectly gaslight casings, the possibility of gasseeping in and being ignited by a spark would not be a problem. Unfortunately, seals onthe rotating shafts of motors or on the operating rods of solenoids are not gastight.Hermetic sealing of covers is not guaranteed: gaskets or jointing can deteriorate andface-to-face joints depend on quality of the machined surfaces and tightness of fastenings.Inaccurate threads on screwed cover joints or conduit can allow leakage of gas; so too candeteriorating cement or sealant on inspection windows or insulators.

The designers of flameproof equipment recognise that there is a risk of ignition andexplosion of gas within some enclosures and counter this by making the casings strongenough to withstand such an explosion. Also, potential flame paths to the outside of thecasing through joints or seals are made so as to extinguish any flame. Normally this is donein joints by making them face-to-face with a limited gap (Figure 9.3). Note that thepossible flame path between the surfaces is made long enough to prevent flame emission(lengths for different types of joints are given in the construction standards), and packingor jointing is not used in the actual flameproof joint. The flame path through shaft seals iselongated by using a labyrinth design. Special consideration is given to cover fasteningswhich must be strong enough to withstand internal explosion and, as far as possible,gastight. Screws of special matterial must have a unique design so that ordinary screwscannot be used to replace them. Their heads are patterned for special keys. Screw holesmust not pierce the casing.

The glass of inspection windows must be held against the casing by an internal clampingring. Flame paths are incorporated and sealing cement must be durable.

Flameproof enclosures ('explosion proof is the term applied in the USA) are used forequipment where sparking or arcing occurs during normal operation, as in a switch orstarter. The spark is contained and likewise any flame or explosion (of gas which mightenter the casing), preventing ignition of a surrounding explosive atmosphere.

c o v e r jo in t

c o v e r

int ide

i n s i d e

i n s i d e

g l a s s

Figure 9.3 Flame paths in Ex d equipment

UEL


over-rich

level of oxygentoo low forcombustion

mixture too*LEL 'ean 'or combustion

I i I i I i

oxygenin air

16 12 8

oxygen(%)

Figure 9.1 Flammability of air and oil vapour mixtures

type, the momentary glow of a broken light bulb filament, and arcing from a broken ordamaged power cable are more apparent as sources of ignition.

Conventional equipment and cable are suitable for areas that are considered safe.Special regulations apply to hazardous zones requiring only safe equipment or none at all.An unusual condition (as with a gas leak in the home) can make a safe area potentiallydangerous so that all machinery and equipment must be shut down.

Safe electrical equipment

The in ormation needed when selecting suitable safe apparatus for use in hazardous areasis sho\ /n below as on a nameplate (Figure 9.2). The data consists of letter codes andreferences, which in the example apply to flameproof equipment. Other types of protectionalso use the marking for gas grouping and temperature.

The standard to which the safe apparatus is made may be British Standard BS 5501 oran equivalent from the IEC (International Electrotechnical Commission), or anotherstandard. For flameproof equipment, details of construction method can be found in Part

:5 of BS 5501. This item is indicated on the nameplate by (1) together with the code letter forflameproof apparatus (d).

The stylised crown with inscribed letters Ex is the symbol of the testing and certifyingauthority and this, indicated here by (2), is the British Approvals Service for ElectricalEquipment in Flammable Atmospheres (BASEEFA). There are others. The 'Ex'represents the general term explosion proof, which is used in Europe for the safe types ofelectrical equipment.

Reference (3) shows the IEC grouping of apparatus with respect to certain gases. Theroman numeral I is used for underground mining work, where methane and coal dustconstitute the risks. Roman numeral II is used for equipment in industries other thanmining. A sub-grouping for gas types is shown by a letter after the numeral. Thus group

122 Safe electrical equipment for hazardous areas

0

0

\ IBS 5501 Pt 5 d IIB T4

BAScEFA No. Ex 2833010

Figure 9.2 Nameplate for equipment to be used in hazardous areas

IIC is typified by the gas hydrogen, which requires an ignition energy of 29 microjoules.Group IIB is typified by ethylene, which requires an ignition energy of 69 microjoules.Group IIA is typified by propane which requires a higher ignition energy of 180microjoules. This last gas type is obviously the most difficult to ignite. Further details ofthe gas classification can be found from the IEC publication or from BS 5345 Pt 1 (1976).Some countries have maintained their own grouping systems, which are slightly differentand could be confusing.

The energy for ignition can be derived from a spark or from contact with a hot surface.Protection against sparks is necessary whenever flammable gas and air mixtures arepresent. Hot surfaces, however, will not cause combustion unless their temperaturereaches the ignition temperature of the gas. (Ignition temperature is higher than flashpoint.A spark can cause combustion at flashpoint, but a hot surface will only cause combustionat the higher ignition temperature.) Maximum surface temperature classification is givenby the T symbol, reference (4). The table gives the relationship between class number andthe maximum surface temperature.

Class Maximum surface temp. (°C)T, 450T, 300T3 200T4 135T5 100T6 85

Higher surface temperatures are shown as actual figures. Corrections are necessary ifambient temperature exceeds 40 °C or if equipment is in contact with other hot surfaces.

Equipment for hazardous areas is selected so that its maximum surface workingtemperature is less than the ignition temperature of any gas or vapour likely to be found inits hazard environment. Details of gas ignition temperatures and the T classifications canalso be found in BS 5345 Pt 1 (1976) Basic Requirements, which is the UK code of practicefor safe electrical installation in hazardous areas.


The maintenance of light fittings, switchboxes and other equipment which is designedand type-tested as flameproof must be thorough to preserve the 'as tested' condition.Inspection may reveal impairment of structure which would permit gas to enter, and alsoallow flame from gas ignited inside to reach the surrounding atmosphere without beingcooled or extinguished.

Casings or terminal boxes may be corroded, cracked, broken or not properly closed byreason of missing or slack screws or nuts. Glass may be cracked, broken or not adequatelycement-sealed. Deterioration of conduit, cable and cable boxes, or glands may be evident;also rubbing contact between fans or couplings and guards. Deterioration, wear ordamage must be made good by replacement or repair followed by correct assembly aftercleaning of flanges. Feeler gauges are used to ascertain that there is no gap in flangesgreater than the specified maximum (which may be 0.15 mm). Any weatherproofing mustbe fixed as specified; paint is applied with care (not incendive aluminium paint) to protectagainst corrosion without affecting the flameproofing.

Electrical checks are for earth continuity and the satisfactory operation of anyprotective overload devices which are important for preventing overheating. Lamp ratingand type must be checked in flameproof light fittings.

Increased safety (Ex e) equipment

A squirrel-cage induction motor is a type of electrical apparatus which does not normallyhave any associated arcing or sparking during operation and where running temperatureis not excessive. Such equipment can be made safe for operation in areas made hazardousby the likelihood of flammable vapour, by the use of increased safety techniques.

Insulation for windings and cables is of high quality and protection is given againstingress of water or solids which could cause insulation breakdown. Breakdown ofinsulation due to overheating from overload is prevented by overload devices, which arean essential part of the increased safety technique. Adequate clearance is given to the fanand rotor to avoid mechanical sparks from rubbing contact and the casing is madeimpact-resistant. Power supply terminals are of non-loosening type and well separated toprevent tracking and the cables are firmly supported. The overload devices which protectthe insulation also prevent excessive external or internal temperature.

Pressurised (Ex p) equipment

Some deck lights used for tankers are operated by compressed air turbines which drivesmall individual generators within the fitting to provide power for the lamp. The exhaustair pressurises and purges the fitting, so excluding any flammable gas which might bepresent in the external atmosphere. Failure of the air supply automatically causes thepower to be switched off.

The technique of pressurising is also used in straightforward types of electricalapparatus, particularly where it is necessary to install a non-standard piece of equipmentin a hazardous area. Ex p equipment is not permitted to be installed in very hazardousareas. Alarms and automatic shutdown at loss of pressurisation are required if normallysparking Ex p type apparatus is installed where a flammable atmosphere is likely to occur.

Pressurisation has been used for control cabinets and enclosed spaces for safecontainment of sparking electrical equipment. All pressurised spaces must be thoroughlypurged before the equipment is switched on.

Safe electrical equipment for hazardous areas \ 25

Non-incendive (Ex n or N) equipment

The difficulty of absolutely scaling casings against the slow inward seepage of hazardousvapour which is permanently present in the surrounding atmosphere has been mentionedin the section on flameproof equipment. In an area where gas might only be present for alimited length of time, if at all, the problem of accumulation of vapour within a casing isnot so serious: Thus the Ex n protection that can be used with lights in such slightlyhazardous areas, for example, calls for (1) a conventional casing seal, (2) a surfacetemperature limit, and (3) protection against the ignition by operational sparks of anyvapour which reaches the inside of the enclosure. Spark risk is negated by an enclosed-arcspark break device, or by ensuring that the spark is of low energy, or by hermetic sealing ofthe spark location.

Other methods of protection

Equipment for normal power operation can also be protected by oil immersion (Ex o),powder filling (Ex q), or by a special method of protection (Ex s).

Intrinsic safely (Ex i)

The effect of an electric arc or spark on flammable vapour is that the heat increases theenergy of the vapour and air locally so that particles are activated and made ready forchemical combination. The energisation reaches a sufficient pitch after a very small timedelay so that combustion results.

A weak spark or arc may not have sufficient thermal energy to heat the local flammablemixture at a rate faster than that at which the energy is dissipated to the furthersurrounding atmosphere. Without the rise of energy, no combustion reaction will occur. Aweak spark or arc with a slow rate of heating the mixture may not persist long enough tocomplete the ignition at the end of the delay period.

An intrinsically safe circuit is one that is designed for a power so low that any spark orthermal effect produced by it, whether there is a fault or not, is incapable of igniting thesurrounding flammable gas or vapour. It follows that intrinsically safe equipment is usedin such circuits and is designed on the same basis, i.e. of being unable to produce a sparkwith enough power to ignite the specific flammable vapour or gas involved. Intrinsic safetytechnique requires not only that a system is designed for operation with very low power,but also that it is made invulnerable to high external energies and other effects.

Intrinsically safe apparatus is currently made to two standards of safety. Ex i(a) is thesymbol for the higher standard, which requires that safety is maintained with up to twofaults. This type of equipment can be fitted in any hazardous area. The other standard isgiven the symbol Ex i(b) and apparatus made to this specification is safe with up to onefaults The Ex i(b) products are not used in the most hazardous areas. Manufacturers ofintrinsically safe apparatus state that this method of protection is suitable for electricalsupplies at less than 30 volts and 50 milliamps. It is used extensively for instrumentationand some control functions.

Care is exercised in design that capacitance and inductance within the electricalinstallation are kept to a minimum, to prevent storage of energy which in the event of afault could generate an incendive spark. Ex i systems are isolated from other electricalsupplies even to the extent that the cables are not permitted to be in the same trays as those


X X

•upply

1

Ml* «••

.PP..,-

— w

A

1

TVVV'

•* 'k^75n.rT-̂ ^

'YVVV

^L.

*-

i .1Int. Ml*

appv»tu»

v, \ „ r ^IFigure 9.4 Safely barrier for Ex i equipment

of other cables (to prevent induction effects). Systems are earthed and protection isprovided by inclusion of shunt diode safety barriers between hazardous and non-hazardous areas (Figure 9.4). The safety barriers have current-limiting resistors andvoltage bypassing zener diodes to prevent excessive electrical energies from reaching thehazardous areas.

Neither certification nor marking is necessary if none of the following values areexceeded in a device: 1.2 V, 0.1 A, 20 microjoules, 25 milliwatts. However, great caution isneeded when deciding whether apparatus will operate within all of these limits and anyassociated system would have to be certified as intrinsically safe.

Protection related to types of rotating machinery

With most types of electrical apparatus, a choice can be made as to the particular methodof protection that is most suitable and economical. Four methods of explosion protectionsuitable for rotating machines are described below, together with any limitations. Noelectric motors or other rotating machines with these types of protection are permitted inpurnprooms or other similar ha/arclous enclosed areas of ships, or in the most hazardous(zone 0: see page 129) areas of any onshore or offshore installation. Equipment withnon-sparking Fx n/N protection can, in fact, only be positioned in zone 2 or leastha/.uriious areas.

1. Flameproof (Ex d) protection

Flameproof Ex d protection can be applied to any type of rotating electrical machine but isintended for those where the ignition of a flammable atmosphere is likely because sparksor arcing occur during-normal operation of the machine. Wound rotor induction motorswith slip-rings and brashes, also commutator motors, are, because of sparking, likely to beprotected by a flameproof (Ex d) enclosure. With such protection, a particular piece ofequipment may be acceptable for zone 1 and zone 2 areas but the flameproof protectiondoes not make it suitable for the most hazardous zone 0 locations.


Explosion test

The casing for flameproof protection of an electric motor or other type of rotatingmachinery must be strong enough to withstand pressure developed during an internalexplosion. The strength of a prototype of the casing is put to the test by the sparkplugignition of an explosive mixture containing a specified gas. The pressure developed by theexplosion is found and may then be used as the basis for a further static or dynamic test at1.5 times the explosion pressure or 3.5 bar, whichever is greater. For the protection to bedeemed satisfactory, the casing should not have been damaged or deformed. (It is thecasing or enclosure which is tested by the explosion and which must remain intact, butthere is no stipulation about internal damage.)

Flameproof test

The requirement that the enclosure must also prevent transmission of any flame from aninternal explosion to any explosive atmosphere surrounding the enclosure is tested byrepeating the explosion test at least five times with the machine in a chamber which is filledwith the same explosive mixture. If the mixture in the chamber is not ignited, then thesecond set of tests is considered to have been passed and a Flameproof Ex d certificate isgranted. Recertification is needed for any modification of the casing.

Explosive mixtures vary in their ability to force flame through a gap of givendimensions; machines are grouped according to the design of joint (Figure 9.3) withrespect to length of flame path, maximum gap and dimensions. Gases are listed for a givenenclosure group.

General comments

A flameproof enclosure need not be hermetically sealed: emphasis is placed on design ofpotential flame paths for effectiveness in preventing passage of the flame, rather thanproviding a seal between any covers and the casing. With no jointing material permittedbetween cover and casing, the face-to-face gap, although limited in size, will allow theentry of small amounts of flammable gas or vapour to the enclosure. As in a Davey lamp,where a small quantity of gas burns in the presence of the flame, so in a flameproofenclosure for a machine with arcing contacts the gas will normally burn at a slow rate inthe area where arcing occurs. If a larger quantity (within the explosive limits) accumulates,there may be a contained explosion.

There is no limit imposed by the flameproof (Ex d) requirements on the rating of amachine or its internal temperature other than what may be involved in limiting theexternal surface temperature of the enclosure or may be imposed by the rating of theinsulation material.

2. Increased safety (Ex e) protection

Increased safety (Ex e) protection, as applied to an electric motor, is described in thesection dealing with Ex e equipment (page 124).

3. Pressurised (Ex p) protection

Protection by pressurising is acceptable for all types of rotating machinery includingmotors with commutators or slip-rings. A requirement for protection by pressurising isthat the casing is purged or scavenged so that any gas that may be present is completely


displaced before the power is switched on. Air is normally used for pressurising butnitrogen or other inert gas may be employed.

The adequacy of pressurisation is tested by connecting a manometer (glass U-tube inwhich water is displaced by the internal pressure).

Efficiency of scavenging is also tested. The procedure involves displacing theatmosphere (air) within the enclosure with a safe, easily available test gas which has anappropriate vapour density and then, in turn, using the intended purging/pressurising gasto displace the test gas. A 'before and after' analysis of gas samples taken from a number ofmonitoring points is used to verify the purging.

This type of protection also involves a surface temperature classification.

4. Non-sparking (Ex N) protection

This type of protection can be used for squirrel-cage or brushless synchronous machines.The features are similar to those for Ex e protection but less demanding. The criteria for ExN protection may well be met by standard machines. Certification is required, however,but the type of protection only makes the machine suitable for zone 2 areas.

Tanker installations

Regulations and practices applied to the installation of electrical equipment in tankersspecify the types of safe equipment that can be fitted in the areas where flammable gas andair mixtures may be present. The degree of risk is not the same throughout the hazardousareas (Figure 9.5), which include cargo tanks and the spaces above them, pumproorns,cofferdams and any closed or semi-enclosed spaces with direct access to a dangerous zone.

Cargo tanks are permitted to have only intrinsically safe apparatus which is certified tothe higher Ex i(a) standard.

Pumproom lighting must be flameproof (Ex d) and arranged with two separate andindependent circuits. Intrinsically safe apparatus to Ex i(a) standard is also allowed.

Cofferdams adjacent to cargo tanks can be fitted with intrinsically safe equipment.Deck areas above cargo tanks can be fitted with a wider variety of certified safe

equipment including intrinsically safe apparatus and others such as flameproof (Ex d),

3 m. from openings

pump room

Figure 9.5 General tanker arrangement showing hazardous and normally safe areas


increased safety (Ex e), and pressurised (Ex p), depending on distance from tank openingsetc.

Forecastle spaces are considered hazardous if the entrance is within the area of deckwhich is hazardous and only certain safe types of equipment are permitted.

Selection of safe equipment for all tanker installations must take into considerationclassification as to gas type (e.g. 11A - least incendive) and ignition temperature in relationto surface working temperature of the equipment.

Exceptionally, submerged electric motors are fitted in the cargo tanks of liquefied gascarriers. Provided that there is liquid in the tank, there will always be boil-off vapour atsufficient pressure to exclude air. Cargo pump motors are isolated when gas-freeing or atany time when there is a danger that air might enter the tank. Automatic shutdown andalarm in the event of low liquid level is required (initiated by low liquid level, low motorcurrent or drop in pump discharge pressure).

Installations ashore

Codes of practice in the oil industry generally classify risk areas for the likelihood offlammable gas/air mixtures being present as:

Zone 0 - Explosive gas-air mixture continuously present or present for long periods.(Only intrinsically safe apparatus, certified to the higher Ex i(a) standard, is permittedwhere there is a continuous hazard.)Zone 1 - Explosive gas-air mixture likely to occur during normal operation. (Equipmentcertified as intrinsically safe, Ex i(a) or (b); flameproof, Ex d; increased safety, Ex e;pressurised, Ex p; or having special protection, Ex s; is allowed in areas of intermittenthazard.)Zone 2 - Explosive gas-air mixture not likely to occur and any occurrence would be for ashort time. (Areas hazardous only under abnormal conditions are usually freelyventilated, and all of the safe types of equipment are permitted provided that they conformin other respects.)

There are variations in zone classification due to different codes of practice, methods ofinterpretation and number of zones (some countries recognise only two). The use of zoneclassification has been suggestediTor tankers.

>.

Reference

Towle, L. C. (1985) 'Intrinsically safe installations on ships and offshore structures'. Trans.I.Mar.E.S 8, Paper 4.

CHAPTER TEN

Shaft-driven Generators

Auxiliary diesel-driven generators which run continuously for twenty-four hours a dayboth at sea and in port can be expensive in terms of the fuel cost and maintenancerequirement. Maintenance is usually based on running hours which, with continuousoperation, will add up to 720 per month or over 8000 in a year. Even where economy isachieved by the use of a blend of cheap residual with the more expensive distillate fuel, theaccumulation of running hours still gives a maintenance requirement and it is possible thatthe workload will be heavier due to problems with the fuel.

A generator drive taken from the main propulsion system provides the means ofreducing maintenance by avoiding the use of auxiliary diesel at sea. It also furnishes amethod of obtaining electrical power from the cheapest fuel. Additional advantagesbrought by the installation of a shaft generator are that fewer diesel generator sets areneeded and the shaft-driven machine can be of large enough capacity to take the full at-seaelectrical load.

Power for the bow thrust while manoeuvring is provided exclusively by the mainengine-driven alternator on some ships, with power for auxiliaries being provided at thattime by two diesel sets. At sea, when the bow thrust is shut down and auxiliary load istransferred to the main engine-driven alternator, the diesel machines are stopped.

General arrangements

A generator positioned directly in the shaft line between the main engine and propeller(Figure 10.4) can be built so that its shaft is flange coupled as part of the intermediate shaftsystem, or the rotor can be based on a split hub which is clamped to a section of the mainshaft. The outer part of the generator is supported on the tank top. The problem with thisarrangement is that the air gap will vary due to hull flexure and weardown of bearings andan excessive clearance of perhaps 6mm may be required.

Another arrangement, developed by the builders of large slow-speed diesels, has alarge-diameter multi-pole short-length alternator (Figure 10.1) mounted on the forwardend of the main engiiie. The projection adds only about one metre to the engine length andwith the casing bolted to the engine, and rotor to the crankshaft, no extra support isneeded and an excessive air gap is not required. With both schemes described above, thegenerator runs at what is likely to be a low engine speed.

Shaft-driven generators 131

large diametershort-lengthalternator

Figure 10.1 Large slow-speed main engine with an alternator at the forward no-drive end

Vee-belt drives have been used to step up d.c. generator speeds, but for alternators astep-up drive through gears is more usual. The drive can be taken from the intermediateshaft (Figure 10.2a) or from a main gearbox or from the engine itself (Figure 10.2b), toprovide higher speed. The power take-off (PTO) from the engine through gears may befrom the camshaft or crankshaft.

Another frequently used option (see Figure 10.5) to obtain higher speed is that ofcoupling generators to the non-drive end of the medium-speed main engines. Theadvantage of this arrangement is that the generator ran be of large power because themedium-speed main engine is a prime mover that can be disconnected from the gearboxand used in port. Large-capacity cargo pumps for tankers or dredge pumps can besupplied with electrical power by such a system. The type of arrangement shownincorporates a high-voltage switchboard and distribution system which has been favouredfor many installations.

gear box

main engine

(a)

shaftgenerator

Figure 10.2* Generator drive from the intermediate shaft

132 Shaft-driven generators

main engine

(b)

Figure 10.2b Generator drive by power take-off from the main engine

gear box

Speed variation

With d.c. systems the use of shaft generators was common because of the acceptability oflimited speed variation. Fluctuations in speed of d.c. generators tend to cause change ofvoltage but this can be corrected by appropriate design of the excitation system and theincorporation of a voltage regulator.

The 60 Hz (or in some cases 50 Hz) frequency of an a.c. system is required to bemaintained within very close limits; the speed of any main propulsion-driven alternatormust therefore be kept within a very narrow band by the speed governor of its primemover. One method for obtaining a.c. power from a shaft drive (Figure 10.3) utilised abelt-driven d.c. generator (with automatic voltage regulator) which supplied a d.c. motor,the latter being coupled to provide a mechanical drive to an alternator. The field of theshaft-driven d.c. generator is separately excited using power from the ship's three-phasea.c. supply. The excitation from the switchboard is delivered through a three-phaserectifier to the field as direct current. The AVR uses the output from the tacho-generator tomonitor d.c. drive motor speed and thus the voltage output of the d.c. generator. The lattermust be kept constant to maintain constant speed of the d.c. motor and the alternator thatit drives.

The arrangement can accept and operate satisfactorily with a main engine speedvariation of ±15%. When reducing main engine speed, the changeover from shaftgenerator to diesel drive may be automatic or manual.

The avoidance of speed deviation is achieved for many main propulsion-drivenalternators by opting for a constant-speed engine with a controllable-pitch propeller. Asuitable engine governor is essential.

Most ships have fixed-pitch propellers so that ship speed variation necessitateschanging engine revolutions. To accommodate changes of engine and shaft alternatorspeed, a constant-speed power take-off may be installed or, more usually, output from thealternator is delivered to the electrical system through a static converter. The converteraccepts a range of generated frequency but delivers a supply at the frequency required bythe system. Static frequency converters have been developed for use with shaft alternatorswhere the speed range extends from 40% to 100% of the rated speed of the main engine.

Shaft-driven generators 133

d.c. loop protectivecircuit breakei

separatelyexcited

generatorfield self-excited

motor field

feedbackloop

rectifier unit

440V 3-phasesupply to

switchboard

440V 3-phase a.c. supply

mamswitchboard

Figure 10J Alternator driven by shaft-driven d.c. generator and motor

Static frequency converter for a shaft generator

The converter system shown (Figure 10.4) serves the shaft generator of a ship with afixed-pitch propeller and a large main-engine speed range. The shaft generator mustsupply full output over the permitted speed range, and to achieve this at the lower end (i.e.down to 40% of the rated speed), it is overrated for higher speeds.

The a.c. shaft generator itself is a synchronous machine which produces alternatingcurrent with a frequency that is dictated by variations in engine speed. At the full ratedr.p.m., frequency may match that of the electrical system.

The output is delivered to the static converter, which has two main parts. The first is arectifier bridge to change shaft generator output from alternating to direct current. Thesecond part is an inverter to change the d.c. back to alternating current, at the correctfrequency.

Alternating current from the shaft generator, when delivered to the three-phase rectifierbridge, passes through the diodes in the forward direction only, as a direct current (see ~Figure 2.12, page 25). The smoothing reactor reduces ripple. The original frequency(within the limits) is unimportant once the supply has been altered to d.c. by the rectifier.

The inverter for transposition of the temporary direct current back to alternating


main switchboard bus-bars

clutchcontrol

alternator andsynchronouscompensator

mam engine

Figure 10.4 Shaft generator system with static frequency converter

current is a bridge made up of six thyristors. Direct current, available to the thyristorbridge, is blocked unless the thyristors are triggered or fired by gate signal Gate signals arecontrolled to switch each thyristor on in sequence, to pass a pulse of current. The patternof alternate current flow and break constitutes an approximation to a three-phasealternating current.

Voltage and frequency of the inverter supply to the a.c. system must be kept constantwithin limits. These characteristics are controlled for a normal alternator by the automaticvoltage regulator and the governor of the prime mover, respectively. They could becontrolled for the shaft alternator inverter by a separate diesel-driven synchronousalternator running in parallel. The extra alternator could also supply other effectsnecessary to the proper functioning of an inverter, but the objective of gaining fuel andmaintenance economy with a shaft alternator would be lost. Fortunately the benefits canbe obtained from a synchronous compensator (sometimes termed a synchronouscondenser), which does not require a prime mover or dr iving motor except for starting.The compensator may be an exclus ive device w i t h its own starter motor or it may be anordinary alternator with a clutch on the drive shaft from the prime mover.

The a.c. generator set that fulf i ls the role of synciiionous compensator for the systemshown (Figure 10.4) is at the top right of the sketch. The diesel prime mover for thecompensator is started and used to bring it up to speed for connection to the switchboard.The excitation is then set to provide the reactive power, and finally the clutch is opened,

Shafl-driven generators 135

air-operated clutchwith flexible coupling

gearbox

=0=medium-speed

main engine600 r.p.m.

415Vswitchboard 3.3RV

switchboard

alternator

frequencyconverter

transformer[dredge pump

3300V bow thruster

Figure 10.5 Shaft generator supplying a high voltage system

the diesel shut down and the synchronous machine then continues to rotate independentlylike a synchronous motor, at a speed corresponding to the frequency of the a.c. system.

A synchronous compensator is used with the monitoring and controlling system, todictate or define the frequency. It also maintains constant a.c. system voltage, damps anyharmonics and meets the reactive power requirements of the system and converter, as wellas supplying, in the event of a short circuit, the current necessary to operate trips.

The cooling arrangements for static frequency converters include the provision of fansas well as the necessary heat sinks for thyristors.

High voltage system for a dredger

The necessity for high electrical power for dredge pumps, bow thrusters and othermachinery has led to an increased investment in high voltage systems. The funct ion of themain engine of many vessels is now divided between propulsion and that of acting as primemover for generators with a large power output for auxiliary machinery.

The diagram of the main propulsion and electrical supply scheme for a dredger (Figure10.5) illustrates the changing role of the main engine. In this arrangement, when the vesselis manoeuvring or dredging and using less propulsive power, the excess is made availableto the bow thrust or dredge pump. Full engine power is used only when on passage

References

Gundlach, H. (1972) 'Static frequency converters for shaft generator systems'. Trans. I. Mur.F..Hcnsel, W. (1984) 'Hnergy saving in ships' power supplies'. Trans.I.Mar.E. 96, Paper 49Mikkelsen, G. (1984) 'Auxiliary power generation in today's ships'. Trans.I.Mar.E. 96, Paper 52.


Murrell, P. W., and Barclay, L: (W84)~ 'Shaft "driven generators for marine application*.Trans.l.Mar.E. 96, Paper 50.

Pringlc, G. G. (1982) 'Economic power generation at sea: the constant speed shaft driven generator'.Trans.l.Mar.E. 94, Paper 30.

Schneider, P. (1984) 'Production of auxiliary energy by the main engine'. Trans.I.Mar.E. 96, Paper51.

CHAPTER ELEVEN

Electric Propulsion

An electric propulsion arrangement for a ship is often simply described as a diesel electricor turbo-electric system. It is characterised only by the type of prime mover, with noreference to the type of electric propulsion motor, the generator or the electrical powersystem.

The electrical side of the system will be based on a direct current or an alternatingcurrent motor, coupled to the ship's propeller shaft, with speed and direction of propellerrotation being governed by electrical control of the motor itself or by alterations of thepower supply. An electric motor used with a controllable-pitch propeller is arranged foreither constant or variable speed operation.

For a direct current (d.c.) propulsion motor, the electrical power may be from one ormore d.c. generators or it may be alternator derived and delivered through a rectifier as ad.c. supply. The rectification scheme can incorporate speed control and the means ofreversing.

Power for an alternating current (a.c.) propulsion motor is supplied by an alternatingcurrent generator (alternator). The prime mover that provides the generator drive may bea diesel engine, a gas turbine or a boiler and steam turbine installation.

Electric propulsion has been u$ed mainly for specialised vessels rather than for cargoships in general. These include dridgers, tugs, trawlers, lighthouse tenders, cable ships, icebreakers, research ships, fioatingtcranes and vessels for the offshore industry. The mainadvantage lies with the flexibility and absence of physical constraints on machinery layout.Support ships for the offshore industry, particularly those with two submerged hulls, canuse electric propulsion motors to give high propulsive power in the restricted pontoonspace, while generators and their prime movers are housed in the large platform machineryspace. Electric power can be used for the self-positioning thrusters and other equipment, asindicated in Figure 11.7, as well as for main propulsion.

Passenger ships with electric propulsion benefit because the number of generators inoperation can be matched to the speed and power required.

Advantages of electric propulsion

The 1< rge amount of electric power available for main propulsion can be diverted for cargoor dn dge pump operation as well as for bow or stern thrusters and fire pumps of theemerj ency and support vessel (ESV) described. There is potential for reduction in the sizeof propulsion machinery spaces, because machinery is smaller and the generators, whether

I 38 Electric propulsion

Diesel, steam or gas turbine driven, can be located anywhere. One proposal for a liquefiednatural gas carrier was for a pair of gas turbine driven generators located at deck level,with electric propulsion motors of 36000h.p. (27000kW) situated in a very small aftmachinery space. Electric propulsion separates the shaft and propeller system from thedirect effect of a diesel prime mover and from transmitted torsional vibrations.

"Disadvantages of electric propulsion

Higher installation costs and lower efficiency compared with a diesel propulsion systemare the likely penalties.

D.c. propulsion motors

The power for direct current motors is limited to about 8 MW so that a.c. machines areused for higher outputs unless resort is made to the installation of d.c. motors in tandem.

Ward-Leonard control

The Ward-Leonard system, as described in Chapter 8, has an a.c. induction motor drivingits direct current generator. Field current for both generator and motor is deliveredthrough a three-phase rectifier from the a.c. supply.

The simple Ward-Leonard arrangement for diesel electric propulsion (Figure 11.1) is anall-d.c. scheme with a diesel engine as the prime mover driving the single d.c. generator atconstant speed. An exciter mounted on an extension of the generator shaft provides fieldcurrent both for the generator and for the direct current propulsion motor. The exciter isitself a d.c. shunt generator.

At start-up, the armature windings of the exciter have current generated in them whenthey pass through the field emanating from the residual magnetism of the exciter poles.The small current generated in i t i a l ly , circulates through the windings of the exciter poles,strengthening their magnetic fields unt i l ful l output is reached. The current generated inthe d.c. exciter is delivered unchanged to its own field poles and to the field poles of the d.c.

shunt_J /-^X |~field <|^v/d,c._M ,^ ) (generator) g

threadedrods

variableresistance

Figure 11.1 Simple Ward-Leonard system for diesel-eleciric propulsion

Electric propulsion 139

propulsion motor. It is available to the field poles of the generator, but only through theregulating resistances of the manoeuvring control. If the control contacts are at the midpositions of the resistances, then no current flows to the main generator poles and there isno output from it to the propulsion motor. Rotation of the manoeuvring handwheel andgears turns the threaded bars to move the contacts along the resistances, in oppositedirections. As the contacts travel toward the extremities and resistance lessens, currentfrom the exciter flows to the generator field poles. The direction of current flow and thelevel are used to control the output of the generator and, in turn, the propulsion motor.Propeller speed is proportional to the actual voltage produced in the generator and fed tothe propulsion motor.

D.c. constant current system

The generator of the simple Ward-Leonard system described above is dedicated to thepropulsion system because control of the propeller is based on variation of the generatorfield current and voltage output. One advantage of the constant current and other electricpropulsion systems is that the generator is involved not with control, but solely withsupplying power. The two 610 kW d.c. generators of the constant current system shown(Figure 11.2) supply power at a constant current of 1000 amps for the bow thruster as wellas for the two propulsion motors. Other equipment designed for the particular powercould also be supplied.

ter

fi j_</ 1 working1 amplifier -

1

o — I

standbyamplifier

~ Q exciters Q

S__

b

refe

<TUT

ence

) ^yuTT^ /TSUIS> <r~z> d — b 1 (1 )s startxaard startioard ̂ — /"5^ generator motor ' \

&

bow thrust motor port generator

Figure 11.2 Constant current (tliesel) electric propulsion system designed b\ W. II. Allen Sons anilCo. Ltd in 1966

140 Electric propulsion

Excitation for the propulsion generators and motors is provided by either of twomotor-driven five-unit exciter sets. Each setconsists of a propulsion generator exciter, twopropulsion motor exciters arid a srnafl High-frequency alternator, all driven by a d.c.motor. The generator exciter can excite one or both generators, simultaneously.Independent control of speed and direction for propulsion motors requires separateexcitation, hence the two propulsion motor exciters. The 400 Hz alternator supplies amagnetic amplifier for constant current control.

Constant current systems, as described, were installed in two small heavy-loadroll-on/roll-off vessels which were built in 1966 and which at the time of writing are still inservice.

D.c. motor supplied from a.c. generators

Direct current propulsion motors installed in more recently built ships are supplied withpower from alternators through control and rectification systems. The basic arrangement(Figure 11.3) shows a diesel-driven high voltage a.c. generator with an a.c. exciter on anextension of the shaft.

d.c.circuit-breaker

shaft-mountedexciter 1

reversible fieldsupply

Figure 11.3 D.c. motor drive from alternator

a.c.generator

dieselprimemover

Output from the three-phase exciter is rectified and delivered to the alternator rotor asdirect current. The level of this current is controlled by an AYR to maintain constantvoltage output of the a.c. generator.

The alternating current output from the a.c. generator is delivered to the d.c. propulsionmotor armature through a thyristor bridge, as direct current. Control of the gate signalsfor the silicon-controlled rectifiers (Figure 11.4) alters the level of voltage and hence speedof the motor. The smoothing reactor reduces ripple.

Reversal of the propulsion motor is effected by changing the direction of direct currentthrough the field poles.

appliecLvoltage


smoothed d.c.

smoothed d.c

smoothed d.c.

time

Figure Hfl Control of power to propulsion motor

A.C. propulsion motors

The necessary reversal and speed changes essential for a synchronous motor coupled to afixed-pitch propeller are obtained in the classic turbo-electric installation (Figure 11.5) byswitching two phases of the three-phase power supply to the motor and by altering thespeed of the steam turbine, respectively. With this scheme, the variable-speed a.c.generator and '-he electric propulsion motor provide a system which is a substitute for agearbox. Manoeuvring is partly by electrical and partly by mechanical means.

The arrangement allows flexibility in the positioning of equipment, but the change ofspeed and frequency of the turbine-driven alternator is essential to the control ofpropulsion motor speed. This means that the alternator must be dedicated to thepropulsion motor and cannot be used simultaneously to supply power to other motors.The drawback of having the generator involved with control (as with the d.c.Ward-Leonard system) is avoided when constant-speed alternators are used either withconstant-speed motors and controllable-pitch propellers, or with propulsion motors withvariable speed and direction coupled to fixed-pitch propellers. Control of propulsionmotors is provided by modern solid state equipment and generator power can be used forthruster, pumps and other auxiliary machinery.


reversingcontacts

synchronouspropulsion

motor

condenser

high-pressurewater tube

boiler

Figure 11.5 Basic turbo-electric propulsion system

Turbo-electric propulsion

A turbo-electric propulsion arrangement (Figures 11.5 and 11.6) can be used as thealternative to a reduction gearbox for coupling a steam turbine which is most efficient athigh speed and a propeller that provides best results at low speeds. The fleet of T2 tankersbuilt as the tanker equivalent of the Liberty ships during the 1939-45 war had a propulsionsystem based on a steam-turbine-driven generator, which supplied power to an electricpropulsion motor coupled to the propeller. At a time when a rapid tanker-buildingprogramme was essential to the war effort, the adoption of an electrical drive overcame theproblem of developing a larger reduction gearbox and building up the manufacturingcapacity. An advantage of turbo-electric propulsion is that an astern turbine is notrequired, as reversing is effected through the switchgear.

The component parts of the simple turbo-electric installation are the propulsion motorwhich is coupled to the propeller shaft, the turbine-drL: >n generator, and the boiler whichsupplies steam for the turbine. The synchronous generator, intended for high revolutions,has a two-pole-wound rotor and is of relatively small diameter to reduce problems due tocentrifugal effect. Generator excitation must be capable of continuous control to providehigh excitation and voltage for starting and a varying excitation, as well as voltage for thedifferent speeds and frequencies. The synchronous propulsion motor runs at a slow speed(say 106 r.p.m.) to suit propeller efficiency and has, therefore, a large number of salientpoles. Direct current for rotor excitation is provided separately. The propulsion motor hascopper bars and end rings in the periphery for starting as an induction motor.

The turbine is warmed through in the normal way and, at the first movement followingstand-by, the control valve is opened to admit steam and bring the generator set to idlingspeed. When the direction contacts are set, the alternator is provided with extra excitationto raise its voltage and the propulsion unit, now temporarily an induction motor, isaccelerated to idling speed. The torque of an induction motor at starting varies inproportion to the square of the applied voltage. The propulsion motor now becomessynchronous as the rotor poles are supplied with direct excitation current. The generatorexcitation is now reduced and kept to an appropriate level as the speed of the turbine andpropulsion system is increased as required.


motor

asterncontacts


motor


a.c.generator steam

static turbineexcitation

earthing resistor

f\— earth£; faultC — alarm

~ earthed neutralpoint

steamstatic turbine

excitation

propulsionmotor

excitation

Figure 11.6 Basic electrical scheme for turbo-electric propulsion

— earthed neutra1

point

Prior to stopping/reversing, the system is brought back to idling speed and theexcitation is cut off.

The main propulsion alternators (Figure H-.6) rraveearthed neutrals, with resistance tolimit any earth faul t current and alarms.

A.c. one-speed drive with C.P. propeller

Controllable-pitch propellers are used to provide speed variation and manoeuvringcapability when one-speed synchronous or induction (asynchronous) propulsion motorsare installed. The former are more costly than the asynchronous types but also moreefficient. Synchronous motors do not diminish the power factor of systems and by increaseof their excitation can remedy the decrease of p.f. caused by other equipment.

Because of this non-consumption of reactive power and better efficiency, the generatorsupplying a synchronous motor does not need to be as large as for an induction motor of


equivalent size. However, induction motors have simple and more robust rotors and norequirement for an electrical supply to the moving rotor, and they need less maintenance.

The necessarily small air gap between the rotor and stator of an induction (asyn-chronous) motor is a disadvantage because the propulsion motor shaft will flex with theship's hull . The air gap for a synchronous motor is larger and such a motor is thereforemore suited to the task.

The high starting current demanded by both synchronous motors (which may bestarted as induction motors) and induction motors themselves may make necessary theuse of auto-transformer, series inductance or some other starting method. Direct on-linestarting is acceptable for many propulsion motors.

A.c. induction motor drive with C.P. propellers

The electrical power system of many contemporary vessels encompasses electricpropulsion and the supplies to pumps, thrusters and other equipment. The generators arenow no longer auxiliaries but the main providers of power for all purposes. An example isgiven with the power arrangement (Figure 11.7) for a platform ESV.

The 6.6 kV three-phase three-wire system (with insulated neutral) is supplied withelectric power by up to six 3.4 MW diesel generators. The HT switchboard provideselectric power for two propulsion motors, two combined propulsion/fire pump motorsand two exclusive fire pump motors, each of 2.24 MW. In addition, there are four thrustermotors of 1.5 MW, and other supplies for auxiliary purposes via transformers. Clutchespermit flexibility with combined duty and main propulsion motors.

Control of the comprehensive installation has been simplified by two features. One is theuse of direct on-line-start induction motors for main propulsion and thrusters system. Theother is that controllable-pitch propellers are employed for main propulsion and thethrusters.

Fixed-speed a.c. generators with variable-speed synchronous motors

The potential for using static electronic equipment to change an electrical supply can beseen from the conversion systems described in Chapter 10 and from the description of ana.c. drive for a d.c. propulsion motor in this chapter. Static frequency converters are usedin a number of ship's installations (Figure 11.8) as the controlling intermediary betweenfixed-speed alternators and variable-speed synchronous propulsion motors. The outputfrom the a.c. generators is delivered at constant voltage and frequency, but formanoeuvring or slow speeds is passed on to propulsion motors at a lower frequency andwith voltage adjusted. The speed of a synchronous motor is dictated by the frequency ofthe current supplied.

Many synchronous drives are based on conversion of the output from fixed-speed a.c.generators first to direct current and then back to a.c. at a lower frequency (the opposite ofthe converter scheme for variable-speed shaft generators). The vessel, when operating atfull speed, will receive power at normal frequency and voltage straight from theswitchboard.

Cycloconverter

The cycloconverter method of controlling speed also relies on the ability of the converterto accept current from the switchboard at constant frequency and voltage but to pass this


portpropeller

starboardpropeller

emergencyswitchboard

starboard 415V,boiler

transformer415V switchboard

(main)6.6kV switchboard

Figure 11.7 Electrical power arrangement for an offshore emergency and supply vessel

] 46 Kleclric propulsion

static frequencyconvener


.motor

constant-speeda.c. generator

Figure 11.8 Synchronous motor electric drive

current to the a.c. motor at a reduced frequency and with voltage adjusted. Thecycloconverter is different in that it operates without an intermediate d.c. stage in theconversion.

The fixed-frequency supply from the a.c. generators is applied simultaneously to thethree pairs of Graetz thyristor bridges (Figure 11.9.* of the cycloconverter. The upper andlower bridges of each pair are arranged to operate alternately (Figure 11.10) so that anumber of triggerings occur in the top set of thyristors - followed by an equal number fromthe bottom set, to deliver an output with a lower frequency. The two bridges for each phaseare required to supply both the positive and negative half-cycles.

The frequency pattern is shown very simply to illustrate the principle. There is greatervariation in reality because the triggering of the thyristors is continually changed relativeto the three-phase supply so that output can be tailored to provide the exact frequency andamplitude of voltage required. Frequency is variable from 0 to 60 Hz.

The windings of the propulsion motor shown in the sketch are separate from each otherto maintain electrical isolation between phases. If they are to be connected as a commonwinding, transformers are required at the input from the switchboard to each pair ofbridges.

References

Gibbons. D. }., and Rulhcrford, K. J. (1980) 'Machinery system design for a platform emergency andsupport vessel' Trans.l.Mar.E. 92. Paper 10.

Rush, H., and Taylor, S. K. (1982) 'Electrical design concepts and philosophy for an emergency andsupport vessel'. Trans.l.Mar.E. 94, Paper 28.

Smith, K. S., Yacamirii, R., and Williamson, A. C. 'Cycloconverter drives for ship propulsion'.Trans.l.Mar.E. (paper presented December 1992).

Taylor, S. K., and Williams, J. S. (1985) 'Design considerations for electric propulsion of specialistoffshore vessels'. Trans.l.Mar.E. 97, Paper 6.



motor

cycloconverters

Figure 11.9 Cycloconverter propulsion motor control

upper bridge operating

lower bridge operating

Figure 11.10 Frequency change using a Cycloconverter

CHAPTER TWELVE

Miscellaneous Items

Fuses

High current flow through a thin fuse wire will raise its temperature, causing it to melt andbreak the circuit before the current excess reaches a level sufficient to damage other, moresubstantial parts of the system. Meltingtemperaturedepends on the material used (tinnedcopper in rewireable fuses melts at 1080 °C, the silver in cartridge fuses at 960 °C). The wireis sized so that the normal current is carried without overheating but, due to the resistanceof the relatively small wire, that excess current will produce heat sufficient to melt it.Current rating gives the normal current that may be carried; minimum fusing current is thesmallest current that will cause melting.

A fuse will melt much quicker with very large fault current than when the value of faultcurrent is only just above the minimum fusing current. Time/current characteristics arefound by testing six or more of the same type of fuse at different currents and plotting theresults. The bottom current for the test is not more than 1.05 x minimum fusing current,and the top current is one' 'iat will melt the wire in not more than 0.5 of a second. The othertest currents are equally spaced between these. Fuses are rated for particular a.c. and/ord.c. voltages.

Cartridge fuses

High rupture capacity (HRC) fuses have silver wire enclosed in a quartz-powder-filledceramic tube with metal end-caps (Figure 12.1). Arcing when this type of fuse blows isburied in the powder, fusion of which in the arc path helps to extinguish it.

silver fuse

tell-tale

K Y^X \ x \ \ \ \> "* '

ceramic case

Figure 12.1 HRC or High Rupture Capacity fuse

Miscellaneous items 149

HRC fuses can be used for very high fault levels; deterioration is negligible; they haveaccurate time/current characteristics and reliability for discrimination; they are safer ifaccidentally inserted on a fault; there is no issue of smoke or flame; cartridges are sized toensui e that the correct value fuse is fitted.

Semi -enclosed fuses

The rewireable fuse has an insulated carrier for safe handling and containment of the wirein an asbestos-lined tube (Figure 12.2). The wire is easily replaced after operation, but thedesign is open to abuse as too heavy a wire can be used which could mask a fault and alsocause severe arcing if it did operate. Another fault is that of premature failure if the wire ismade thinner by oxidation or contact with air, or by being stretched when fitted (aproblem with wire made of lead, tin or an alloy of the two).

fuse wire asbestos tube

copper contacts

Figure 12.2 Rewireable fuse

Fuses in service

Fuses may be used as the only protection in a steady-load circuit, such as for lighting. Ana.c. motor with its high starting current and varying load has fuses in each of the supplyconductors, but fitted as a back-up for the other forms of protection and to break thecircuit in the event of a short-circuit current greater than that which the ordinary contactbreakers are designed to interrupt without damage. Very accurate time/current character-istics are needed for fuses used in conjunction with other safety devices, to ensure that theoverload trip is allowed time to operate for moderate over-current but that the fuse blowsfirst if there is very high short-circuit current.

Incandescent lamps

High current flow in the coiled high-resistance tungsten filament of an incandescent lampgives a temperature rise sufficient to produce an almost white glow. Much of the electricalenergy supplied, however, is radiated in the form of heat and less than 20% is converted tolight. Best efficiency jnight be obtained with a filament temperature of over 6000 °C;however the best filament material, tungsten, melts at 3400 °C. Above 6200°C efficiencydrops as the wavelength of the radiated energy shortens to the extent that "non-visibleultraviolet light is emitted at the expense of visible light.

A near-white-hot filament would quickly burn away in air (as seen when a light bulbbreaks), but vacuum bulbs tend to blacken from evaporation of the tungsten. Introduction

150 Miscellaneous items

of the inert gases argon and nitrogen lengthened filament life by reducing evaporation andallowed higher temperature operation and greater efficiency.

Disadvantages of incandescent (filament) lamps compared with the fluorescent type arethese. (1) They are more expensive to run, giving less light for the power supplied. (2) Theyproduce about twice the heat for the same light output. (3>^ilament glare is a problem inclear bulbs for lighting although it may be an advantage for indicator bulbs. Pearl bulbsare less efficient but give a more even light. (4) Filament lamps have a shorter life and areaffected by shock and vibration. (5) Voltage reduction will reduce filament temperatureand increase life, but efficiency drops. Higher voltage improves light but shortens life.Rough service lamps are available.

An advantage of incandescent lamps is that they do not require a complicated startingcircuit and this makes them convenient and cheap for many purposes.

Fluorescent lamps

The inner surface of a fluorescent lamp tube is coated with fluorescent powder. Light isproduced from absorption by the (phosphor) powder of ultraviolet radiation andre-emission of part of this energy as visible light. The ultraviolet radiation comes from theeffect of an electrical discharge on a low-pressure mixture of mercury vapour and argon,between cathodes of tungsten coated with thermionic emitter. Emitter material isconsumed at a very slow rate when the light is on but each start uses a relatively largeamount. Useful life of the lamp ends when emitter is depleted.

Switch start circuit

A glow type starter is a switch consisting of bimetal contacts separated by a small gap andcontained in a gas-filled bulb (Figure 12.3). Circuit voltage is high enough when switchedon to produce a glow in the gas and sufficient heat to make the bimetal contacts touch as a

glow starter

| — ̂ =I acoil

-f—MQs-

tube phosphor coating

b<argon and mercury vapour

cathode

supplyI

capacitorfor power (actor improvement

Figure 12.3 Fluorescent lamp with glow-stun circuit

Miscellaneous items ,151

result of differential expansion. Once the contacts come together, arcing across the gapceases and the glow is extinguished. The closed contacts now complete a circuit throughthe inductance coil and cathodes, causing the latter to heat up to red heat. The circuit isbroken when the cooling bimetal contacts move apart. This interruption of current in theinductance coil creates a high-voltage pulse that ionises the gas and vapour in the tube andinitiates current flow between the cathodes. With current flowing, voltage drops to belowthe level needed to re-ignite the glow bulb.

A thermal starter switch (Figure 12.4) operates on the same principle, in that it opens toproduce high voltage by inductance. It also has bimetal contacts but these are closed whencold. When the circuit is energised, current flows through coil, heater and cathodes.Heating caused by the current brings the cathodes to red heat and the switch heater to atemperature high enough to open the bimetal contacts. Interruption of the current in thehighly inductive circuit produces the striking voltage to start the lamp.

coil—OLJJU-

supply

thermal switch

capacitor for powerfactor improvement

Figure 12.4 Thermal switch start circuit for fluorescent lamp

Shore electrical supply

The connection box for taking an electrical supply from ashore is fitted in a position whichis easily accessible for the cables and itself connected to the engineroom switchboard by apermanent cable installation. A convenient place may be a locker at deck level that can bereached from either side of the vessel. Figure 12.5 shows equipment for taking an a.c.supply.

Details of the ship's a.c. system, with voltage and frequency, are given in the connectionprocedure instruction in the terminal box. A ship-to-shore earth is required if the shorepower is three-phase with an earthed neutral.

After terminal connections are made, before closing the breaker, phase sequence ischecked with the indicator to ensure that motors will not run in the wrong direction. Theindicator lamp shows availability of shore power at the switchboard and the supplybreaker is closed after the ship's alternators are disconnected. Overload protection isprovided by the supply breaker or an isolating switch and fuses. There is usually a lamp toshow that the supply is on, together with a voltmeter and ammeter.


to switchboard

circuituin,uu 0| 0, 0.breaker I I I

o l o l o lindicator lamp

phasesequenceIndicator

shore supply connection

Figure 12.5 Arrangement for taking a.c. shore supply

Moving-coir meters

Current supplied to a conductor lying in and at right angles to a magnetic field will set up amagnetic field around the conductor which will react with the main flux and tend to movethe conductor out of the field. The operation of a moving-coil meter relies on this principle,as does the electric motor.

The field of a moving-coil meter is provided by a permanent magnet with the flux beingstrengthened by a soft iron core fixed in the gap by clamps top and bottom. A moving coil,wound on a light frame and mounted on a spindle, is fitted so that the coil sides can rotatein the space between the iron core and the poles. The spindle passes through a hole in theiron core and is supported in bearings at either end. Current from the supply to be testedpasses in and out of the coil via hair springs (non-magnetic) which also control themovement. The small angle through which the coil moves is proportional to current flowthrough the coil and a pointer on the spindle indicates the reading on an even scale.Current in the coil is limited by the small size of the wire to perhaps 69 mA, so that metersintended for use as ammeters measuring current require a low-resistance shunt to bypassthe greater part. A resistance in series is required by moving-coil instruments intended foruse as voltmeters.

Moving-coil instruments are basically devices for measuring direct current, and theiruse with alternating current requires that the supply be rectified. Oscillation of the pointerdue to current fluctuations or spring vibration is damped by eddy current induced in thelight aluminium frame on which the coil is wound.

Moving-coil ammeter

Small current can be measured with a moving-coil instrument without the necessity of ashunt, but otherwise one is needed to bypass the excess current. Shunt size is fixed in


meters for switchboards and starter boxes. Test ammeters have several built-in shuntsarranged with a switch for changing the meter range, and some have external replaceableshunts.

A shunt must be capable of carrying heavy current without overheating and itsresistance must not change appreciably with temperature. False readings are likely if theammeter is connected to an external shunt with leads of different resistance to thosesupplied.

Mojdng^caiLvoltmeter

Resistance in series with the moving coil is inserted when an instrument is intended for useas a voltmeter. Usually the resistance is mounted inside the instrument. A multi-rangemeter has a tapped resistance with a selector switch to change the operating range.

The resistance, sometimes called a voltage multiplier or multiplier resistor, preventslarge current flow through the instrument, which has only a fine winding on the movingcoil. Thus a voltmeter can be connected across the terminals of a 220 volt d.c. generatorand the large series resistance will reduce current flow to the level suitable for the meter.Voltage variations will vary the amount of current passing through the resistance and themoving-coil meter will register the changes on a scale marked in volts.

Moving-iron meters

These meters are versatile instruments that can be used for measurement of current orvoltage with both d.c. and a.c. equipment.

The repulsion type has a large coil with terminals for connection of the supply to betested. Current in the coil, whether d.c. or a.c., will set up a magnetic field so that the fixedand moving irons within the field are also magnetised. Both irons are magnetised with thesame polarity so that they tend to repel each other. The moving iron, being attachedthrough a lever to the pointer spindle, causes the instrument to register the effect on a scale.

Pointer movement is controlled by hairsprings on the spindle ends, but these meters areoften shown in sketches with weights for gravity control. Pointer oscillation is reduced bythe air-damping vane.

fElectric shock

The effects of severe electric shock and immediate first aid required for its victims areshown by posters on display at high-risk areas such as the switchboard. Resuscitationtechniques are also taught in the mandatory first aid courses. Certain conditions increasethe dangers from electric shock, and risks are greater when using portable a.c. appliancesthan with fixed electrical installations.

Current from a steady d.c. source, in passing through the skin, will tend to causemuscular contraction at the initial contact and as contact is broken. Alternating currentproduces a continuing spasm in the muscles through which current passes, with its changefrom forward to reverse flow at the rate of 50 or 60 cycles per second. Alternating currenthas the additional ability to stimulate nerves directly. Most victims of 'serious shock' willhave been in contact with a.c. Serious shock results in unconsciousness or worse, requiringresuscitation and medical care.

Alternating current which takes a path through the chest area can, by contraction of the

154 Miscellaneous Hems

chest and diaphragm muscles, stop the breathing directly, and possibly also indirectly byinterfering with the operation of the respiratory control nerves. Similarly, shock in theregion of the chest can have direct consequences for the heart, causing stoppage fromcontraction of the heart muscles. Lesser alternating currents can upset the heart'spumping action by destroying the coordination between the walls of the ventricles(ventricular fibrillation). Current flowing through the body can cause clotting withinblood vessels so that tissues are starved of blood. Various nerves may be affected, also thebrain or other vital organs could be injured.

Serious shock as a consequence of the above can kill instantly, in so far as stoppage ofthe heart and breathing are equated with death. However, with the power shut off or theperson safely removed from contact, the prompt and continuing application of first aid hasa 75% chance of saving life. (With shock, arrest of breathing and heartbeat are not theresult of a physical defect but of a temporary condition induced by the electric current, andwith only brief contact there may not be serious damage from the current.) Resuscitationto overcome loss of heartbeat and breathing calls for both heart massage and artificialrespiration to be employed. An unconscious person who is not breathing must be givenartificial respiration. After recovery, victims of shock are kept under close observationbecause of the likelihood of relapse. Unconsciousness or other forms of distress may bedelayed and not follow immediately after a shock which has apparently left the victim onlyshaken.

Burns

Damage from electrical burns may not appear to be extensive from the surface mark(sometimes just a small wiiiicned area), but the penetration may be deep. Current flow can-2UoC liuumg of the blood and destruction of tissue. Most cases of severe burning resultfrom contact with a direct current supply.

Conditions which increase danger to personnel

The involuntary spasm caused by electrical contact on some parts of the body sometimesmakes the victim jump away. Contraction in muscles of the hand caused by contact witha.c. can mean there is inability to release the object from which shock is being received, andso contact is prolonged. A current of 12 to 15 mA or more through the muscles is sufficientto make relaxation of the grip impossible and a 10mA current can be fatal over a longperiod.

The resistance of dry skin to current flow is fairly high, but that of wet skin much less (thebody's internal resistance is very low). Thus in warm conditions danger from shock isgreater due to sweat on the skin and this has been a feature of some welding accidents. Theresistance of wet skin, if taken as 10000 per cm2, would permit current flow from a 220Vsupply (1 cm contact area) of 220/1000 = 220mA This is more than enough to be lethal.Obviously a higher voltage would increase the current flow. Other factors which reduceresistance of the skin are poor general health and cuts or other surface damage.

Current flows into the body through the part in contact with a live conductor and thenout through another part which is touching earth or another live contact at differentpotential. The current path may be from one hand to the other, through the chest(resistance between tin- hands may be 2(XK)n depending on the area of skin involved), orfrom hand to foot etc. Current flow into the body is less when the sk in is dry; and if there is


resistance on the current path between the body and earth this will further reduce orprevent current flow and shock (rubber mats, dry metal-free footwear). There is greaterrisk when working with electrical equipment in humid or wet conditions; in hot conditionswhere skin, clothing and even protective leather gauntlets become soaked with perspir-ation; and when in contact with metal platforms, railings, machinery or a metalworkbench.

The effect of electric shock is more serious for someone in poor health with, say, a heartproblem.

Shock risk with portable a.c. appliances

Faulty hand-held tools powered by a.c. and having metal casings could impart a lethalshock to the operator where the fault causes the casing to be live'. The hand(s) grippingthe tool provide a large contact area (possibly damp with perspiration) so that sufficientalternating current might flow to prevent relaxation of the hold, and such a current couldresult in fatality. The risk of shock is increased if the operator is working in dampconditions and standing on metal plates or touching metal structure. There are similarrisks with various types of portable or semi-portable appliances - particularly lead lamps.

The metal casings of portable appliances are connected to earth through the earth wirein the three-core cable and the earth pin in the plug, to give protection against a fault whichcould make the casing live. Frequently, rough handling of portable equipment not onlycauses the fault which makes the casing live, but also causes the earth wire to be broken.Thus, when electrical connections and insulation are checked in the course of regularinspection and cleaning, the earth eore of the electric table should also be tested forcontinuity (i.e. with one terminal of the tester on the metal casing of the appliance and theother on the earth pift of the plug).

Shock risk from portable tools is greatly reduced if the power supply is taken from thesecondary winding of a transformer used to step down the mains supply to a suitable lowervoltage, with the mid-point of the secondary winding earthed. If the secondary voltage islimited to 110V for operation of the single-phase appliance, then the potential shockvoltage between the casing and earth is limited to 55V. (Secondary voltage can be madelower if required.) Double-pole switches are fitted to control single-phase appliances fed inthis way.

Flexible cable for portable tools and equipment is reinforced and given extra protectionby a rubber tube where it enters the appliance. Here and at the plug end the cable is subjectto bending and pulling: it can also be damaged along its length by being pinched or cut bysharp edges and by touching a hot surface or lying in oil, chemical or water. Sometimes thecable is cut by the the tool being used. Damage to the cable can cause shock in a number ofways, or an earth or a short-circuit.

During cable inspections for damage to the insulation and continuity (particularly inthe earth wire), other checks are also made. Live and neutral wires must be correctly fittedto terminal points so that the switch is on the live side and the appliance is isolated from thepower supply when switched off. Switch operation is tested. Fuses should be exposed toensure they are of the right size.

Extension leads must be arranged so that, when plugged into the power supply, the freeend has a socket for the three-pin plug of the power tool. If fitted wrongly with a plug, thefree end of the extension lead would be highly dangerous.

A small shock from a portable tool can sometimes cause a fall and in ju ry (e.g. tosomeone working on a ladder).


Merchant Shipping Notice M752 (Safety - Electric Shock Hazard in the Use of ElectricArc Welding Plant) points out the risks involved with the use of arc-welding equipment inhot, damp conditions within a restricted space surrounded by the earthed steel structure ofthe ship. Three fatalities are cited, each resulting from use of a.c. sets with an open voltageof 70 V or so. The Notice states that d.c. welding sets of the same voltage are safer (unlessderived from a.c. and having unsmoothed ripple). Availability of voltage reduction safetydevices for installation with a.c. welding plant to make it safer is stressed. These safetydevices limit the open-circuit voltage to 25V until electrode contact is made to strike thearc and then full open-circuit voltage is turned on. The Notice also recommends the use offully insulated electrode holders, adequate protective clothing including non-conductingrubber boots, good lighting and the display of first aid instructions on a wall chart.Operational advice is given, in particular with regard to handling electrodes, which shouldnot be inserted into a live holder.

Index

Alternatorprotection, 55rotor construction, 41stator construction, 38

Armaturereaction, 95windings, 91

Auto-transformer starting, 72

Batterycharging, 7installations, 10

Bridge rectifier, 24Brushless alternator, 47

Carbon pile regulator, 44Compound d.c. generator, 89Constant current propulsion system, 1:

-Cyclo-oon verier, 144Cylindrical rotor, 41

Diode safety barrier, 126Direct on-line starting, 70Double cage rotor, 75

Earth lamps(a.c.), 61(d.c.), 106

Electric shock, 153Emergency

batteries, 8electrical power, 12generator, 10synchroscope, 60

Equalising connection, 104Excitation systems, 44

Flammable atmospheres, 120Flameproof equipment, 123, 126Fleming's Right Hand Rule, 35 83Fluorescent lamps, ISOFull wave rectification, 23Fuses, 148

Generator protection, 100

Half wave rectification, 22

Incandescent lamps, 149Increased safetey equipment, 124Induction motor, 66Instantaneous short circuit trip, 101Instrument transformers, 63Interpoles (d.c.), 95Intrinsic safety, 125Inverse definite minimum time relay, 56

Lap winding, 91Lead acid cell, 1Loss of residual magnetism, 97

Main circuit breaker(a.c.), 53(d.c.), 99

Maintenance d.c. motors, 118Motor protection (a.c.), 79

Neutral point earthing, 37Nickel cadmium batteries, 3

Overload trip, 101

Parallel operation of d.c. generators, 104Pole change motors, 76

158 lndt:\

Power lac tor . V*Preferent ia l t r ips , 102Pressurised (F.x p) e q u i p m e n t . 124 . 12Propulsion motors U c }. 141Propulsion motors (d.c.) . 138

•Rectification, 22Restoration of residual magnet i sm, 97Reverse current t r ip , 103Reverse power protection. 57

Salient pole rotor, 42Scaled nicke! cadmium cells, 7Self (static) excitat ion system, 49Semi-conductor junct ion rectifiers. 19Semi-conductor materials. 14Series generator, 88Shore electrical supply. 151Shunt generator, 86Shunt motor starting, 109

•Silicon controlled rectifier, 30Single phase induct ion motor. 81Single phasing, 78Soft starting of electrical motors. 74Speed control of d.c. motors, 115Squirrel cage motors, 66Star-delta s t a r t ing , 71

Siatic frequency converter for shaft generator,133

Static automatic voltage regulator, 46Stat ic excitation system, 48Stator construction. 39Synchronous motor. 77Synchroscope, 58

Thermistors, 16Three phase rectification, 24Thyristors, 30Transformers, 61Transient volt din. 50Transistors, 27Turbo-electric propulsion, 142

Under volts release (d.c.), 104

Y_M 'ence electrons, 15Vibrating contact regulator. 45Voltage doubler, 27

Ward-Leonardsystem, 117propulsion system, 138

Wound rotor motor, 74

Zener diode, 25

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